I'm not sure you will find what you're looking for in a hardware solution. Probably the best you can do is a simple USB KVM and you will have to press a button (or perhaps keyboard combination) to switch between machines.
Are you sure there is no network connectivity between the two? if both machines are on the same subnet they should be able to ping each other directly. If they are connected to the same network switch then they're on the same subnet.
The software solution that I have used for this is synergy. They used to be freeware but it looks like they've gone commercial now. Still, it's a cheap software at only $10. The site: Synergy - Mouse and keyboard sharing software
But it does require network access.