here is my situation. i got two xbox360 connected to a router. and of course my pc connected to that router as well. here is the problem, after around 5 minutes of playing, both of my profile will sign out. and i have to sign back in again. what seems to be the problem? and i notice, there are a guest profile.

Both different account & which have guest account? Connected to router, online or offline LAN?

try to check the NAT on your 360s. when i used to have 2 360s one had a moderate and one with open, never both open at the same time. constants DC problems but when only was was connected, the DCs tend to stop.