I usually go for the multi-boot solution. But if I were to do what you
need, I'd just plug in my old PC (a P100), throw in an old hard drive, put
a keyboard switch on the keyboard, and plug the output of the video to the
alternate video input on my monitor (has 15-pin HD VGA connector as well as
BNC inputs). No signal degradation due to video switch, no second monitor
required.
Much easier than the multiprocessor solution, since 1) it works now, 2) most
everyone has or can get a cheap spare PC, 3) no tricky software hackery
required.
FWIW,
Dan Hopper