It is possible to play link-up Doom using only one copy of the game. It's a bit ropey but it does work and I think the main reason for that is that I am doing the swap method on one PSX (ie I have a Japanese and UK Playstation, a PAL copy of the game). Anyway, load up Doom on one of them, choose all the options and then start the linkup. It'll say "waiting for connection". Now wait a second or two, take out the CD and pop it in the other PSX and load it up. Set up the same options, and then start the linkup. It'll say "waiting for connection" then after a while it'll change to "loading" as will the other one. After another while it'll probably just stop. Wait until you can't hear the CD making any more noise than just the sound of inactive spinning and take it out and put it in the first PSX. Now wait a while and it should load up and work. With mine when a level is completed it'll then crash. I think it may be because of the linkup but I don't know. Anyway, it's good while it lasts!