Problems with Multihomed server

I have installed an NT4.0 sp6 server with two NICs. I am only using IP. I have enabled IP forwarding, and installed RIP for IP. My intention is to use this machine as a gateway from one subnet to another 10.1.x.x and 10.2.x.x, both with mask 255.255.0.0. However I cannot route IP across this server. I have tried adding gateways for one, then both NICs (using the other NIC as the gateway address) with no joy. I have even tried using multiple IP addresses on one card, but also to no avail. The server can happilly PING anything, on either side, and a client on either side can ping as far as the NIC in the sever attatched to their subnet. Am I missing something fundamental here?