When you say "I cannot be on both", what does that mean exactly? Does Windows actually indicate one or the other network connection has been disconnected (as in that little network icon in the system tray w/ a red X in it)? Or does it appears connected but any attempt to access a resource just fails? Does it report an error? Does at least one connection continue to work? Is it always the same one? Or do they both fail to work?
IOW, I need more specifics in how it behaves when the problem arises.
And btw, once you have them both connected and the problem occurs, go to Start->Run, type "cmd" (no quotes), and hit enter. In the command window, type "ipconfig /all > temp.txt" (no quotes) and post the file's contents here.