System cannot choose correct Ethernet

Setup: two computers, a hub and an ADSL modem. The sever running Windows 98 SE, the client Windows 98. The Sever has two identical Ethernet 10/100 MBS cards, one connecting directly to the modem, the other to the hub.
Problem: The server cannot differentiate between the two ethernet cards which bot work correctly but show both up in the systems as identical (no #1 or #2 added). When both cards are active and installed cannot log on to Internet (did not even think of trying to log on to client yet).
Question: How can I make this work, how can I make the server differentiate (I already changed the physical location of one card to another slot, no avail).