I've seen problems like this before.
In some cases w2k and xp does not allow the second NIC for sharing your connectin when using cable modems.
I do not know why yet, Microsofts ways can be strange, but it has to do with the way you get your IP. Normally the ISP uses DHCP to give you an IP-adress for the first card. To share the connection you have to set up a "sort of" DHCP-server on your local machine which obviously is not allowed.
You could probably use fixed IP on your local net and get a communication but then the Internet sharing part does not work as it seems to demand the use of DHCP.
I've seen one way around this and that is through the use of a Proxy on your main computer.
My suggestion is this:
* Stop the Internet sharing mechanism
* Set fixed IP on the cards for local communication
* Verify if communication can be achieved this way
If this works, try to find a proxy SW to run on your main computer but beware the problems related to security.