Its true that Port triggers(AKA Application Rules)can help when using two Xbox's, but not for NAT. The only thing needed for open NAT in a game is for 3074 to be forwarded to the Xbox in question. A single port can only be forwarded (or routed) to a single IP. UPnP works by changing that port on outbound to something random that way those two random ports can be routed to the two different IP for the Xbox. Port 88 is a different story though. Your Xbox uses port 88 to sign into XBL. Long story short, the two devices communicate back and forth real fast talking over that port, and sometimes your firewall will block it because the session might be open for the other Xbox as it tries to sign in. Thats why a fix for some issues with two Xboxs are to turn them both off, turn one on, wait, then turn the other one on. If your having THAT issue, or issue with having both Xbox sign in at the same time, then THATS where setting up a Application rule for port 88 can help. But you still need port 3074 for NAT. Make sense?