I have 4 Xbox 360s (2 Wired, 2 Wireless) and 2 PS3s (both wireless) on my network router (Plus 2 wired PCs and 3 wireless Laptops (though not always on at the same time). We also have 2 "Smart" TVs and 2 Bluray players on the wireless (usually just the players are "connected" though) Also at any given time we oan have up to 3 iPads, 4 iTouch, 4 Nentindo DS, and 3 Wiis (very rarely used) hooked up to wireless. The only problem we have is a 7MBps connection LOL
But back to your issue. My wife and I play Call of Duty, ALOT! When we first got online with the new ISP after our move we had lag and disconnect issues when playing CoD togather, but on seporate Xbox. It turned out to be some kind of ping issue because both Xbox were sending the same information packets so it bottlenecked at the ISP. They changed our ping so I was told and ever since its been much better. (outside of normal CoD problems and network issues)
Check with your ISP about your Bandwidth and maybe ping issues. Also it could be a bad female connection on your router/modem, which ever you have the ethernet cables from your Xboxes connected to. Hope this might help or give others some ideas to throw out there. Good luck.