Usually a POV just seems right, but there have been a few times that I started writing from one POV, found it wasn't working, switched to the other POV and it was much better. So I guess I'd say trial and error, which isn't very helpful :D

In one fic for another fandom, I knew from the beginning that I wanted to use the POVs of both main characters, so each section changed POV (and I gave each section its own chapter for clarity). Maybe alternating POVs would work for your fic too?