Actually, a user-defined port-forwarding profile for one xbox and full DMZ on the other doesn't work because when all of the ports are already open for one unit, you can't reopen the ports for a different ip. Why? It doesn't matter. You just can't.
What are these mystical gateway modems that you can adjust your NAT settings on, because I am starting to think it's not a real thing. I've spent days of my life trying to fix this problem.
The person who figures this out is going to be a millionaire.