It's perfectly reasonable to require that every employee be able to get along with both the founders. That doesn't mean kissing ass or not arguing, but it does

It's perfectly reasonable to require that every employee be able to get along with both the founders. That doesn't mean kissing ass or not arguing, but it does mean not being an asshole (as determined by each of the founders). If you want him gone, and your co-founder can't convince you that you don't, there's only one right answer. Your co-founder owes it to you to unconditionally support you on things like this. He should make it very clear to everyone that his loyalty is with you above all.