Because you have them on two different connections to the gateway, Windows file sharing I don't think can be done. They don't see each other directly over a network segment, they only can reach each other by passing data to the server first. You may even have them on entirely different subnets for each NIC.
Windows Networking for file sharing doesn't use IP, it uses a non-routable protocol called NetBIOS. The way systems find each other is by broadcasting, which requires being on a single broadcast domain. Your computer isn't set up to forward such broadcasts from one NIC to the other, and I'm not sure any software routers can do that, but possibly. I think with WinXP (and you must install a WinXP sort of emulating software on a Win98SE machine to allow it to share with WinXP), the NetBIOS traffic is actually passed over IP, but still requires broadcasts to go through.
Look into FreeSCO, and the Linux Router Project. You may be able to find something on their forums about it. You can also check with the makers of Smoothwall and see if there is a way to make broadcasts be forwarded, so the computer ends up acting like a bridge between the two NICs (though you will have to have both computers and NICs set to the same subnet in all likelihood). Finding out if you can allow broadcasts between the two NICs is the first step, maybe the IPs are a non-issue.
You could also map a drive letter between each machine, using their IP, rather than the computer name through Windows Networking.