Well im gonna give up on the subject, in fear of repeating myself or trying the same thing from a different angle; but.
On
this website it can be found that the Microsoft certified routers with the correct firmware can run 2 xbox's with open NAT's; how?
I understand completely what your saying about 1 external address, and the fact they both fight for pole position to get to the open ports, but this been the case whats different with the certified routers,
And
here for the list of certified routers,