By what year would they have been colonized?
The answer to this question is heavily dependent on what resources we are able to find there. In this instance, Venus and Mars would obviously have the great benefit of an additional place that was safe for humans to live.
You'd see the timeline start diverging mid century, where we're starting to get better observations of Venus and Mars. I'm going to make the assumption that you don't want drastic differences to the two planets - Venus still being obscured in mysterious clouds, and Mars still being primarily red.
Going into the 1950s and 60s, we would have some awareness that there was liquid water on Mars - if there are large seas that is, we would probably be able to detect them via telescope.
The point of real divergence happens when these respective probes land on the planets - Venera finds a lush rainforest. Viking finds a desert, but nonetheless, one that shows signs of life.
From here the prediction begins to get fuzzy. In our real history, while significant fractions of the public protested the Moon landings as a distraction, many in the public were confident that we would soon be going to Mars, and were excited about this "inevitable" sci fi future.
The problem is, getting to space is still pretty difficult. The tyranny of the rocket equation makes getting things and people into space expensive, and thus, in the 1970s, with the Vietnam war, an energy crisis, and unrest as we tried to figure out the aftermath of the civil rights movement, congress was very uninterested in exploring space. There is just as much of a likelihood that NASA could've gotten completely dissolved or just greatly diminished rather than what we got.
What got NASA to keep moving was the promise of the space shuttle, a much cheaper, reliable craft to Earth orbit. After establishing this, we would build space stations, and cheaper architecture to return to the Moon, and truly establish an in-space economy.
This however, was not to be. The shuttle was vastly more complex than NASA and its contractors had intended, and after the Challenger and Columbia disasters, it was solidified as having failed in its purpose. It did many great things, but had failed in its goals nonetheless.
Flip flopping priorities from presidential administrations, and lack of interest from congress led to little progress in space until the 2000s. By this time, NASA couldn't really be killed off, because they were providing so many jobs around the country, but neither could they really progress, as a plethora of factors, mostly political, were holding them back.
The thing that really changed the tide were the commercial companies. SpaceX, Rocket Lab, and many others coming onto the stage in the past twenty years really disrupted the industry and have lowered the cost. The true consequences of this I feel we'll only really see by the end of this decade.
By decade's end, we will likely see at least one commercial space station, Axiom being the most likely, but we could have as many as three or four (starlab, orbital reef, Northrup Grumman station).
Meanwhile, NASA is finally getting the political support necessary to return to the Moon, and SpaceX is gunning for Mars. I am personally more optimistic than some in my predictions - and thus I say that we will probably get people on Mars sometime after 2032, and I think it's very likely we will get there before 2050.
The cost of spaceflight is decreasing dramatically, and with commercial space stations, the demand for human spaceflight will also increase, leading to more reliable and cheaper flights for people. Thus, while many engineering challenges lay in getting to Mars, it is for the most part a money problem. We could've tackled the engineering decades ago, I would say. But that money is important - and focus. The commercial companies have shown to be very adept in that regard. Unlike NASA's contractors, they cannot waste money for little development, or else they will go out of business.
Now. Tangent on bringing up to speed the current state of today's spaceflight world, let's return to the original question - how would this be different in this new scenario?
My unfortunate answer is probably not tremendously. The planets being discovered to be habitable would have an immediate effect of excitement in the public - life has been discovered on another planet. You'd probably see congress greenlight more funding for NASA to conduct science missions, especially sending their own missions to Venus.
Actually getting there would be difficult. You'd still have the question many had then, and still today - why explore space when we have enough problems on Earth? This is an overblown feeling by the public being quite frank, considering that the average American pays about $30 a year to NASA in exchange for tons of jobs, scientific discoveries, and NASA developed technologies being spun off into ones useful for consumers.
It honestly is quite up in the air. It all depends on if NASA could get the support behind it to get there faster. They might get trapped in a paralysis of sending many scientific probes, that though making many discoveries, would not be pushing things forward.
Perhaps the commercial sector may spark into existence a bit faster. Whether it would have been as successful is a big question. SpaceX changed the whole paradigm because they wanted to do things quick and dirty. They wanted to break the monopoly that Boeing and Lockheed Martin had on space. Whether a similar company could have arisen a decade or more earlier is up in the air.
What I am quite confident in is that SpaceX's iconic reusable boosters tech likely couldn't have come around too much earlier, as it requires pretty good computers controlling the descent. This would have a pretty big impact on the cost of space. It does not preclude other reusable tech coming about, like gliding boosters down to a runaway.
Whatever the case, for your colonies to come about, you either need a world willing to drop buttloads of money, burning hundreds of millions to billions per launch, or find some way to make a rapid reusable rocket - and one that doesn't require extensive refurbishment like the space shuttle.
The problem is that even if you find something valuable, like rare-earth materials on Mars, getting them back is so prohibitively expensive, it wouldn't be like how gold and spices pushed forward exploration five hundred years ago. It could sustain a colony after a great deal of time, after launch costs get lower, but as for pushing the first exploration closer, probably wouldn't do a lot.
One possible idea is that the population scare, most prominent in the 70s (before improvements in agriculture showed we'd be able to feed a larger population) could perhaps prompt a movement to send people to Venus or Mars. This could bleed into political support for space, with congress giving some token support towards larger NASA budgets.
Conclusion then for when we might see the first boots on Mars and Venus? As I said earlier, probably not too much different to our reality. These planets would be a lot more enticing, but many of the challenges are still there. If there were bigger changes, I could see us getting to Mars in the 2020s or even perhaps earlier, but it'd likely be rather ramshackle. How this pans out depends heavily on what decade it happens in, what the geopolitics are like, etc. You could come up with reasonable explanation for it happening in the 90s maybe, but not too much earlier I'd say. You might look at the alternate history show For All Mankind. Not super accurate, but gives enough explanation for a work of fiction.
Venus would be more difficult, as presumably you want to return home on the early missions, and due to the larger gravity, would require a larger return rocket (which is already a difficult task with Mars exploration). I'm confident in saying then that we would probably see Venus happen later, but not extremely later. It depends on if there is competition between countries or companies who might want to take the "real first prize."
And by what year (if different) would those colonies become self-sufficient?
Oh boy. Self sufficiency. That is a tough question to answer. I'd venture a guess that it would be faster than with our real Mars and Venus. It would be slow going I'd say early on. Much more native flora and fauna would prompt a lot more scientific research before colonization could happen in earnest. It would likely lead to protests from the growing environmental movement, that "we're already screwing up our Earth do we need to screw up two more?"
This is already a thing that a lot of people are saying in our world, and we don't even know if there's anything living out in our solar system. It would be much more pronounced in this world. There is also a lot of risks to consider with space originating diseases, astronauts quarantining after missions, and likely a good while before we'd start seeing people walking around on Venus or Mars without a space suit. It would be at least easier that they would only need something more similar to a hazmat suit, and not a full blown pressurized space suit.
If the environments of these planets proved suitable to human life, if the protesting was managed in some way, then you'd likely start seeing colonization efforts arising. It would depend heavily on if you could grow food in the planets' native soil. Mars in real life has very toxic soil that would need to be heavily processed before use.
This depends on if the life on these planets is carbon based, how similar it is to Earth, etc. If its carbon based, and there are ecosystems on the planets allowing breakdown of soil, we could perhaps grow food there - but that would likely only happen after years of astrobiologists studying this new life.
Your early colonists would, for the most part be scientists. Commercial companies would probably start moving in next. There isn't much of value business wise on these planets still, but Mars would be a good stepping stone to the asteroid belt, where mining could be done.
So I'd venture that Mars would not only be visited first, it would also grow faster. Venus would be a more desirable place to live probably, depending on how hot it is, but it wouldn't have much value beyond the science, unless we discover something valuable there.
So yeah... this scenario gets into a ton of "what ifs." It allows you a lot of flexibility in coming up with reasons why something can happen earlier and still be plausible - perhaps "the spice must flow" on Mars. I'd probably make an upper limit that self sufficiency on both worlds would probably happen prior to 2100 (which I think could probably happen on Mars in our world) but technology could make that go faster, environmental concerns slow it down, or politics just not get its rear in gear.
I can for sure say that this change to these two planets would really muddy up things. People will likely be more persuaded by the idea of becoming a multiplanetary species (a goal often spoken of by SpaceX) - obviously people get the idea of having insurance in case of the absolute worst of Earth facing extinction - but knowing that Venus and Mars are actually livable places would increase this allure a lot more. Most people interested in living on Mars today are... eclectic, lets say. It would be a difficult life, where we would need to spend centuries at least in protected bases or underground before we could have a world where we could breathe. This world though, would be much different.
The environmental concerns though, are again a big factor in how this pans out. Today a lot of political discourse goes around about the impact Europeans had on the natives of the Americas - in the future it might be that future Martians and Venusians looking down upon their ancestors for tramping down the native creatures and plants.
This has been a very, long response, but it is a scenario that prompts a lot of questions. You could take it in a number of ways, have many interesting stories to tell. You could investigate how we deal with being the stewards of another planet when we barely know how to manage our own. You could look at how this changes the culture and ambitions of people a thousand years from now. Maybe we care little about pursuing interstellar travel when we have three whole habitable worlds to contain humanity.
I wish you luck as you study this. For further study, I recommend looking at stuff on space exploration (like from the youtube channels Scott Manley and Everyday Astronaut) and stuff on Astrobiology, like the work of Seth Shostak.