I can't try them as I don't have two Xboxes, but there are instructions on configuring DD-WRT and Tomato firmware based routers to enable two Xboxes in the same connection at
http://jakebillo.com/two-xboxes-one-...-wrt-or-tomato
To see if your router is compatible with dd-wrt (and download it if it is), go to
http://www.dd-wrt.com/site/index, click "router database" and type the model number in to the search box.