If they are both descending, would it try to pick the "better performer"? Each aircraft type given a dexterity score? Interesting puzzle.

I too would think that at least between 2 aircraft equipped with TCAS, one could be given a climb resolution and neither given a descend. I don't know how "intelliigent" TCAS is and if that could be programmed.

I have some but not a lot of familiarity with TCAS, so I'm not able to answer edge cases in any detail. I will say the missed approach safety is only guaranteed if cleared for an instrument approach, not if on a visual approach.

RAs are climb or descend, and descending abruptly below 1000 feet could definitely cause different problems.