Hi David. What you need is a KVM switch with built-in "PS2 Port Emulation" which tricks the PC into thinking the PS2 mouse is still connected when you switch it over to the other PC.
Example here:
http://www.amazon.com/Belkin-2-Port-.../dp/B000087LCG
Or you can get an add-on port emulator to use with your existing KVM switch.
Example here:
http://www.kvm-switches-online.com/cv100km.html