Man, I could've sworned I answered a post like this yesterday. I believe you have to set up one of the computers as a DMZ host if you're planning to host the game. You can also try forwarding port 6112 (I'm not sure how much help that would be, but just give it a shot). I hope you have separate CD-Keys, because having the same one can also be a problem. Hope this helps. 🙂