If you are stuck with a couple coax network cards you can look around for an old ethernet hub with a coax connector and use that (asuming you can buy or make some coax connectors).

I still have an old 3com Office Connect TP4 Combo hub (purchased new in the 90's for my small home network).

The unit has 4 RJ45 ports, coax, and an aui port, so you can pretty much connect anything to it. These old parts should be dirt cheap on ebay.

http://cgi.ebay.com/3Com-OfficeConnect-TP400-4-Port-Ethernet-Hub-3C16723_W0QQitemZ280319469888