The easy answer is "yup! mountains!" the truth is more complex
Sometimes we get caught up too much in "reality." In "reality," this planet cannot exist as you've described. But what's the fun in that? Therefore, I'm going to utterly ignore all aspects of physics that would either (a) tear this planet apart or (b) force it to eventually become spherical. Instead, I'm going to answer the question from the perspective of your world and its rules.
All that mass must go somewhere
Your drawing (nice, BTW) shows about 1/3 of the small planet and 1/5 of the large planet being involved in the collision. All that mass had to go somewhere. A bit of it would be converted to energy during the fissionable component of the collision, but most of it is still there.
Where is it?
Some of it is pushed toward the center of both original masses, actually causing a bit of a dimple on the far side of each mass. So, you have some mountainous regions at both poles due to the collision.
Some of it creates a high-density region extending more-or-less to the center of both original masses. The region is sphericalish. This creates some small (probably not particularly noticable) variations in gravity where it's weaker where you might not expect it, like the southern half of the larger sphere.
Most of it becomes a serious ring of mountains right where you've drawn them. Sink under the oceans? I seriously doubt it! They'd be chain of mountains the circumference of the collision point that would likely challenge Everest for height. Lots-o-force involved with that collision.
But what does that mean for my oceans?
That ring of mountains is not a universal, impassable wall. Clouds will move around and through it. That means rain. Rain means erosion. And erosion means you'll eventually have mountain passes. If the planet is old enough, it's plausible to have "cracks" in that ring of mountains that would connect the waters above with the waters below. However, for the most part, the waters will be segregated.
And my biomes?
The northern and southern biomes will remain fairly segregated for a long time. Birds will get around that, and winds carrying seeds will, too, eventually. The older the planet, the more merged the biomes will become. But they will not be 100% segregated. As a wise man in a dinosaur movie once said, "life finds a way." But it finds it slowly.
Could anything have actually survived the collision?
Nope. At least it's mighty hard to believe. The earthquakes, dust, heat, yada, yada, yada... That was what I consider an extinction-level event. Cockroaches and dandelions might have survived, though.
And what about that magnetic field?
That's not actually how magnetic fields work. They don't follow the contour of solid mass unless that mass is entirely magnetic itself. And even then, they tend to smooth out. What I'm saying is that neat little tuck you show at the seam wouldn't exist. It's a small detail, though, and not very important.
What is important is that magnetospheres are thought to be created due in part to the churning liquid core — which was compromised in the collision. You can't have two independent liquid cores and over time you won't have a core shaped like your surface. Erosion, it would all smooth out inside, possibly even cooling to a thicker mantle at the north pole.
In the worst case, you'll end up with a spherical magnetosphere centered on the original larger mass that does NOT extend far enough beyond the northern pole. Lots of cancer at the north pole! Amazing Aurora Borealis, though.
In the best case the "center of the planet" shifts to something around the original tropic of Cancer1 for the original larger mass. It might very well be weaker (a consequence of that density increase I was talking about, the core is not as fluid as it once was... at least not yet. There's that age thing again).
But, it's your planet, so you might simply define the magnetosphere as you wish it to be!
So could life live there?
Sure! If it evolved after the collision or colonized after the collision. But whatever was on the planet before the collision is burnt popcorn (unless you declare it to be otherwise, it's your world).
The smushed belt of mountains will remain mountains forever. Eventually, erosion would smooth them out and allow some passes to form, but they would never be consumed by the sea. Too much mass involved in the collision.
1 The tropics of Cancer and Capricorn aren't simply the 1/3 points of the sphere, they're defined by the solstice events, which are a consequence of the Earth's axial tilt. I'm using the Earth-reference as a literal map reference for convenience to help you imagine the point I'm talking about. Heck if I know what the original mass' tropics were, or even if it had them.