As far as the theme goes, maybe I didn't get far enough, but it felt like playing as a single character was just as easy as having 2. Maybe add some more mechanics that make it hard to go it alone?
But that's really my only critique. I am kind of blown away by the beautiful art, lighting, animations. The controls feel really good. And it's really fun to play with 2 people. I would also highly recommend full-screen mode.