Wire-networking problem

hi,
there are two similar systems in a room.each with it's own:
- adsl modem(1 usb,1 ethernet port)
-seperate phone line and isp
-onboard network adapter
i want to wire-network these systems and as both pc's are connected to usb port on modems, there is one ethernet port free on each device, please help me to make the right decision ;))
i just want the file sharing and it's better if they can keep their own internet connection. one has windows 7 and the other one runs xp.does it need extra device or it can be done by ip configuration? ( modems are in routing mode not bridge)