I have as of September 17th 2011 successfully connected 2 Xbox's with 1 modem and 1 wireless router.
Please note the only reason I am posting this is because the only semi useful information that is posted is
recycled and incomplete,
Provider: Comcast. (cable)
Modem: Motorola SB6121
Wireless router: Netgear N300
One Xbox is wired all the time and is 3 feet from the modem. The second Xbox is 11 feet from the modem and router, it can
be wired as well. The first Xbox is and Elite and the second is the newer "slim" with wireless built in.
I will update this post if any new information is found to be relevant.
Thank you for reading.