I think there are USB switch boxes out there probably with autoswitching to whichever PC is sending output. Check places like Dalco.com or Computergate.com. All I saw were manual switches, a more thorough search might locate an auto one. Also try jameco.com, cyberguys.com, jdm.com, etc. Of course there's always the printer server solution but that will be a bit more expensive.
.bh.