Fixing first past the post voting is not some magic cure-all.
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.
I feel as a world citizen but I am German; I know the system that was built after the bitter lessons of our history. A proportional representation (with an x% threshold to limit the number of governing parties) is really key to preventing two large parties from dominating everything and becoming the only rivals with the resulting polarisation of the political discourse.
(Free higher education would then be the next step.)