I don't have any solutions, but that's a very good question.

I almost always write in first-person POV, and have struggled with sub-plots in the past. I've usually had to find some convuluted way for my protagonist to find out what's going on, so I can reveal the sub-plot like that. Obviously, that's not usually the best way to go. I'd be interested to see if anyone has any suggestions on this front as well. I'm not a fan of switching POV. It confuses me most of the time, and if I confuse myself writing it, I imagine I'd confuse anyone reading it.