Apparently, Internet Connection Sharing began with Windows 98 so it should handle dual NICs.
What you will need is to configure the two NICs separately, which is not always easy. Study up on the IPCONFIG command.
To see the status of all network interfaces, enter on the command line:
The NIC connected to your cable modem will have had its IP address assigned by DHCP from the cable modem (or local switch/router connected to the modem). It will probably have a private IP address (non-routable) like 192.168.1-254.2-254 or 10.0.0.1-254.
If the other has not been assigned an IP address manually, it should have a default (disconnected) IP address like 165.254.x.x. If that is the case, it will not pass packet traffic and must be manually assigned an IP address.
Use an IP address of the same subnet as your other two and the identical subnet mask. 255.255.255.0 should do the trick.
Default gateway is a problem because your PC will not know where to send packets – through the Internet card or through the LAN NIC. But that is okay, because the default gateway should not be required when you apply a route manually.
You will have to enter the default route for your LAN subnet using the Route command. I’m using a German version of Windows so I can’t copy and paste the help screen. Type the following at the command prompt:
You should be able to get there with the information provided.