I would try it without either first. UPnP should work automatically. If you run into a problem, just try turning off both Xboxes, turn off modem and router, turn on modem, wait until it syncs, turn on router, wait until it syncs, turn on 1 xbox, check that it's online and open NAT, then the other.
Not saying Furry's way doesn't work, because obviously it does, and not even saying my way is even the better way, just saying it is the easier way to try first. If you do it his way, can you let us know? We kind of have a gentleman's wager on something but need someone with 2 xboxes to test it.