Just to clarify, you can "share" it by physically switching between one computer or the other, but you cannot have it actually accessible by both computers at once. All you should need to switch it is a simple switch - you'd need to switch the two data wires, and probably the power wire as well, so a 3PDT or 4PDT switch should do the job if you want to build something yourself.
A quick google search revealed a number of units that do this, most with 2-4 computer connections and 1 device connection, using simple pushbuttons to switch between PC's, but most seem to be $30 or more which seems steep to me.
Personally, I'd just network the PC's, hook the printer up to one, and share it over the network so both can print from it.