connecting 2 PS3 on the same router

[Haiko]

ok I am having trouble on my network when I play World at War with my roommate PS3 giving us an error such as Server full or Game not in session. I know it probably by Upnp which give the 2 PS3 the same ports which wont let us play togerter so is there a way to fix this problem so we can play together?
Here my details
ISP = Comcast
Router= Linksys Wireless-G broadband model WRT54g version 5
1 PS3 is wired the other is in the living room wireless
Thank you for helping out.

[Cephius]

Turning UPNP off would be the first thing I'd try.