a 1/8" mini stereo plug splitter and 2 cables (same plug as a set of headphones use)is what I use.
one of these, http://www.radioshack.com/product/in...ductId=2103870
this will give your speakers 2 inputs.
and a jack to jack extension cable. (2 male ends) http://www.radioshack.com/product/in...ductId=2102949
you will need this cable for the second computer.
this really depends how your speakers are set up. my speakers are powered by the speaker systems amp and hooks to my computer with a single wire.
if yours gets it's signals from the computer through 2,3,4 etc outputs then you will need that many extra cables. it's possible to do but it might get expensive.
any radio shack should have everything you need.