We have 3 xboxes, two 360 S and one 2006 model. Whenever the slim xboxes connect to xbl, there is no problems.
The older xbox can connect to xbl while one of the slim xboxes are connected with no problems, however when it connects xbl while the second slim console is connected it will disconnect the slim console from live.
However if the old xbox is connected to xbl and the affected slim console connects to live, it will disconnect the old console from xbl.
We have tried a few ways to try to fix the problem, we have made sure UPnP is on, we tried setting static IP's to both affected consoles to no avail, we even contacted our ISP to change the network channel, which had no effect.
We just don't know what the cause to this problem is and will appreciate any help you can give.