When you've decided that two characters are indeed POV characters. . .
When they are both in a scene and it turns out, it doesn't actually matter which of them is the point of view. . . they both are affected by what happens, and what they learn, but neither of them has a strong and immediate reaction.
It can be difficult to winkle out which of them would add more with thoughts. Or whether to pick one as the main POV character, or the other just to remind readers it IS multi-POV. . . .