It's this simple. The line you ran to the back garage would go to a small switching hub, then you plug the PC and PS2 into that switching hub. Should be a 20 dollar deal. Here's just one such fixture -> http://www.compgeeks.com/details.asp?invtid=SIL-A-8309&cat=NET There are many more out there.
bob
Hi, my house currently has a linksys going out to 3 computers. One of the computers is out in the back garage but after lots of work we got a line out there. Now we need to get another line for a PS2 online adaptor. I would rather not run a line back there again so I was wondering if it was possible to get another router back there to split the line AGAIN. If it is possible, what is a cheap router I could use?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic