When you say fast interenet do you mean just standard RoadRunner or Turbo?
When you say UBEE Modem do you mean just the modem and you have your own router? Or are you speaking of their wireless router they supply?
UBEE wireless router sucks… Get your own
Running more than one xbox at a time on the same network can be a pain in the ass regardless depending on what your doing. I forgot the feature but some routers have a feature that helps with this by "seperating’ the two xboxes automatically behind the scenes.
I haven’t dicked with it much because I don’t need to… However im not all that suprised.