Easy Solution:
Let me first caveat this solution by saying I am no expert in dealing with manipulating the networks and designating static IP addresses or whatever. This solution has worked well for me and might be an easy way the other people like me to enjoying playing together in the same house/apartment.
The setup that has worked completely reliably for me was to have one Xbox hardwired via Ethernet cable directly to my router and have the second xbox hardwired directly to my modem. My modem is connected via a coax cable connected to the wall and my router is connected via Ethernet cable fed from the modem.
Also - I have moved since I originally had this setup and have had to deal with this issue without having a separate modem. The solution I have worked out now is slightly less reliable but it is simply to have two routers - one for each xbox to hardwire to. This works for ~15 games before requiring a restart for both systems but I've had some issues when in a voice chat party with the other xbox (my girlfriend sits next to me when we play so we just scream out loud instead of in a party to avoid this issue).
Hope this works for some of you!
Best,
C&T + B&B
