I had just edited my above post when you posted this, I touched on the polarizing matchup thing. That's really what I'm hoping for.
I think the majority of the people who main 2 or more characters don't do it so they can CP certain characters, but rather do it so if one character feels off that day (or set or whatever) they can switch. For example, Mango has said many times that he doesn't care what character he's playing against; he will play who he feels like playing. To an extent i think Mew2king can be thrown in this category too (an exception would be his Marth vs fox)
Edit: what I was trying to say with ^^^ is that a lot of people who play more than one char don't necessarily do it for MU purposes
Edit: Also, Im going to have to disagree with you on people playing 2 characters. I think if you want to say that someone plays 2 characters, then they have to be playing their other character at least 25% of the time. These are just random people off the top of my head but PP (goes Marth rarely in pools games for no reason), KK, Armada (played I think one game vs mango with fox at genesis I believe, and also the whole YL thing vs hbox) Kage, Jman, Lucky, Hbox, all the falcon mains, shroomed, javi all really only play one character