2 Games, 1 Connection, 1 Big Issue

Hi,
I’m currently sharing an internet connection with my roommate via a Linksys router. We recently bought the newly release Call of Duty 4, we each have our own copies. The issue at hand is when one of us tries to join the server the other one is on. The person on the server gets a connection interruption. I am at a loss for what to do at this point so any help would be greatly appreciated.
Thank You