We use the MMP system in NZ and it seems to work quite well, though needs a bit of fine tuning as currently you need 5% to get into a coalition and that is slightly too high.
Two party systems also are built on coalitions. It isn't as if "Democrat" or "Labor" is a unified ideology.
I'm all for MMP but I don't think it will fix any problems in American politics.
Just introducing MMP would add another veto point in an American political system that already struggles to actually accomplish anything.