This is to anyone who wants to connect 2 360s on one network and have them both on xbox live playing at the same time. And sorry if someone else has posted this elsewhere and this is a repost.
First. Get a good router, perferably linksys since I did this with one.
When you have the router get into the interface of it, ie log on with the ip address (192.168.1.1-- might vary but should be mentioned in the manuel.)
You are going to want to enable the UPnP setting and just to help out, open the port numbers for the xbox, they can be found online with google.
Then comes the fun part. You need to find out the ip address of the 360s, you actually only need one but it's nice to know both just in case.
At this point you need to enable the DMZ feature in the router. It will ask for an IP Address at which just enter one of the 360s.
Save your settings and you should be good to go.
The reason both 360s wont work, or work a short time, is that the router handling the packets doesnt know which 360 to forward them to so comes the problem with it.