For most of history there would be no difference other than how culture forms.
- Philosophers would probably posit that life on these other planets was not only possible, but evident by their color and atmosphere.
- Scientists might gain valuable insight into their own worlds weather patterns, atmospheric makeup, and other such facts based on their observation of the worlds' twin.
- Religions might form deifying or vilifying the other world (Much like Earth's cultures deify/vilify the moon).
How would this affect technological development in general?
From the start it would definitely drive interest in optics, and efforts to increase lens effectiveness and magnification would have much more motivation. They'll need more and more powerful lenses and telescopes in order to gleam as much information about the other planet as they can through sight alone. This would reveal man-made structures which would betray the existence of intelligent life on the other planet. This would further increase interest and desire for knowledge and communication.
Then as soon as one of the worlds develop electricity, and the surface starts to light up, the worlds would get driven into a research and development frenzy, trying to catch up to their counterpart and out-do them. I wouldn't be surprised to see elaborate light shows to try and impress or communicate with the other world. Rate of discovery and interest in technological innovation would both increase drastically. The next important moment in history would then be when they both develop the capability of sending/receiving signals.
As soon as they're capable of communicating with each other, either through lasers, microwaves, radio, or what have you, that's when the political games will start.
Tensions will be high between the two planets. The leading governments will establish secure lines of communication in order to have political discussions involving the two planets relationship. Whether these talks go well or poorly depends on the personality and character of the delegates from each planet.
In an ideal, peaceful situation both worlds would aid the other and seek to develop interplanetary travel in order to establish trading routes, inter-world tourism, and cultural exchange.
If someone blunders or slights the other, there may be a rat race to develop interplanetary travel -- or interplanetary weapons -- in order to achieve military dominance and control of the planet-system.
Would worlds make areas for transmitting messages and information?
Like I mentioned in the previous section, governments would probably attempt to restrict communication between the worlds in order to limit any possible social or political blunders. As such there might be communication rooms set up in the capitol(s) in order to facilitate discussion (Like an audio-conferencing United Nations).