If you are interested I think I worked out what was going on. I was on the Atmospheric planet (2a) just getting some cool shots
But I noticed when I got close to the collision point I couldn't land because there was no Mass Lock. I moved away until I had it and then landed but when I disembarked from my ship I was floating in air. Relogged and my Ship disappeared but I was still there floating unable to move.
Logged out and when I logged back in an hour later I was floating in black, inside 2a, between the two planets. Relogged a couple of hours later and I was standing on the non atmospheric planet (2b), but my ship was still on 2a
Now comes the cool bit

On 2b I was able to fly right up to where the collision was happening and I could land as I still had Mass Lock.
I think the atmospheric one was travelling around 300m/s away from me. I believe the non atmospheric seems to take preference in the collision. But as soon as you fly to the atmospheric one the mass lock disappears and you can't land. If you fly under the ground of the atmospheric planet you'll see the whole sphere of the non atmospheric one.
So on 2a you can't land within the drop out zone of 2b, I am guessing you need to be more than 50Km away from the collision point.
Hopefully that made some kind of sense