In that case, in order to use both PC's, I THINK you'll either have to pay for another IP adress so you can use the other modem, or use one modem and get a router, then plug both PCs into the router. This is what I've done, and I have PPPoE too. The only problem is, if they are in different rooms, running cables might be a problem. There are wireless routers, but they are expensive and I can't speak for how well they work. And, I haven't really had much experience with the multiple IP thing either, so you might want to check with your ISP for that, or get a second opinion from here.
EDIT: Good luck.