Do you know anything about routing?
You will want to assign one IP to each card, the ip should be from the network that that card is plugged into.
Within the RAS Admin tool, you will want to select the server's name, right-click and get properties. Then on the General tab select "Enable this server as a router" and click "Local routing only". That should cause traffic from one network to route to the other. There really isn't any other necessary configuration to do except to make sure that all the clients know about this server being the gateway.
ShackDaddy