Try looking in somewhere like Maplins for a printer switchbox (can't find any on their web site at the moment) or Dabs (link: click here )
You should be able to find an automatic switch (like the one at Dabs) that only requires you to connect each PC via a parallel cable to the box and a single cable out to the printer. The device should auto sense if both PCs try to print at the same time. Amore expensive solution would be to set up a simple home network with a print server.
Alternatively, for a simple manual solution, try this link: click here
(hope those links work!)
I presume that you don't want to network the PCs and share the printer whilst attached to the host PC as this would entail always having to have the host on-line.