You can totally switch one keyboard between two computers using a 4-pole switch. You could even use a 2-pole switch connected between the USB data lines if you tied all the grounds together and OR'd the power supplies together with diodes (but be careful, check to make sure the is no difference between the grounds and Vccs with a multi-meter before connecting them!).
Each time you throw the switch, you would basically be doing the same thing as unplugging the keyboard from one computer and plugging it into the other.
The downside is that the computers involved will see the keyboard disappear/reappear when you switch, but most operating systems are OK with this and will automatically recognize the keyboard and start using it when it is attached.
(Note this would only likely work for USB 1.0 keyboards. Newer USB needs more wires and is much much faster and will not like going though a switch.)