They sell USB printer switches that might fit the bill for you. Let me search newegg for the model I own, if not I will just snap a few pics. I also have a LinksKey LKV-SO4SK 4 port KVM that is a champ. Works great, but of course it is VGA/PS2 with USB adapters that come with it. BRB with that USB switch.
edit: That was easy:
I have it running a printer between 4 computers but now that I think about your situation this wouldnt work. Maybe search newegg for a solution, or worst case a 2 port KVM.