Welcome to the board, Quory.  If two men both put their penises into your vagina at the same time, of course they're going to feel it.  They both have to be very comfortable with the idea of touching each other, so it sounds like your bf wouldn't like that at all.

I think the best way to experiment with the idea of double penetration is try it with your boyfriend and a dildo.  It would take practice to be able to relax enough to stretch that much. 

If you do decide to do MFM or FMF, I think it's important for all three people to talk about all the things you mentioned to find out what everybody agrees to.  Good communication is essential.