Yes there are S-Video splitter cables. You will degrade the signal strength substantially though. You can buy powered splitters that will do what you want withou the loss, though (for example:
http://www.vpi.us/vsplt-sv.html -- i can't vouch for that, just one i found on google). I use an unamplified y-split cable between my gamecube + projector + computer and it's not too bad, but your mileage may vary. I'll try to find the place where I ordered that cable from, but I suspect you can google for s-video y-split cable and come up with some matches.
ms