If its just a direct NAT problem.. this can occur, yes on the NAT built into the cisco router... but can really happen on ANY router.
There are many people who use a 3rd party router, and sometimes run into the exact same thing.
One thing you could try, is putting the cisco into bridged mode, and then use a 3rd party one that you find doesnt have the NAT problem.
Many people to avoid the NAT issues, is put their xbox/ps3, etc, in the DMZ, so its OUTSIDE of the router, per say.
But in most standard routers.. you can only put ONE device in the DMZ.
Port forwarding.. really isnt useful either... as both xbox's requests OUT appear to be comming from the same location.. no way to know to where to push when they come back.