The computer I am using to type this is going through a KVM switch.  However all the ones I am familiar with are for one Keyboard/Video/Mouse and 2 computers. Son-in-law found this in a Wal-Mart clearance aisle for $15.

What Bryan is wanting here is going the other way.  I would do some playing with multiple inputs to one computer.  For instance run one keyboard and mouse through the PS2 inputs and the other through USB.  Get a video card for a second monitor and run an extension as far as possible.

I've not tried the second monitor, but it is not that uncommon.  Many if not most church's using projectors have 2 monitors on the system.  One is set up for what is coming out the projector, one is used for the system.  I have used 2 keyboards and 2 mice on 1 computer at the same time.  Some computers it works, some it doesn't.