Hopefully this will help someone else out there that had problems with this router and trying to get their NAT to open up for multiple Xbox's. The bottom line is, turn on UPNP and set everything on your consoles to automatic, DHCP,DNS,etc.
I tried everything, DMZ host for one and port forwarding for the other was close but I could never get both to be open. I finally achieved an open NAT on both Xbox's when I turned UPNP on, and did 2 things: 1) I changed the "Advertisemnt Period" from the default(1800) to the maximun,86400 seconds and 2) changed the "Advertisenent Time to Live" from its default (4) to its mas (255). After I rebooted the router I connected both Xbox's and ran the connection test to XBL.....both were now connecting with open NATs. I then went back into the router's UPnP screen and clicked the refresh button for the UPnP Portmap table and instantly saw that both consoles were using port 3074/UDP. This was pretty normal but what wasnt were the other 2 entries. One console opened port 22084 for UDP and the other opened port 10694/UDP. I tested further playing MW3 on one and starting a Netflix app on the other, nothing changed in that table after i refreshed. What is very odd is that, other than 3074, I didn't see any of the ports that we have all been told to open up, 53, 80 and 88.
If anything changes I will post an update here but so far this works.
