I have a laptop running OpenSuSE 11.2. I use the Yast menu to set up network settings via "traditional" ifconfig setup (not NetworkManager), because I found NM unusable in prior releases when doing network gymnastics such as router re-flashing, LAN reconfiguration, etc. where I needed control of the IP address, netmask, DHCP, tftp, etc.

My laptop works fine with Yast "traditional" ifconfig configuration for eth0 connected to router to Internet. Works fine with Yast ifconfig configuration for USB WiFi as wlan1 to WiFi router to Internet. Also works fine for virtual machines on vmnets NAT-ed to the Internet. Works fine as long as there is only one network connection, that is.

Now, I have a second laptop. I want to network laptop #2 eth0 to laptop #1 eth0 for file transfer and/or Internet access WHILE laptop #1 is is linked to the Internet through wlan1.

Unfortunately, I have not figured out how to get Yast/Network Configuration to do that. I have not been able to get Yast/Network Configuration to see both wlan1 and eth0 at the same time--when the Yast utility finishes, I do >ifconfig and >route, and I find that I have lost either eth0 or wlan1. Thus, when I do direct-connect file transfers, I loose the WiFi Internet connection on both machines.

I suspect that it might have something to do with Yast/Network Configuration utility not being able to handle two network interfaces on the host with different IP addresses.

Can anyone tell me how to properly put an OpenSuSE machine on two networks simultaneously? And then how to turn on IP forwarding so that I can use laptop #1's WiFi as an Internet gateway for laptop #2?

As I understand it, you want to make laptop #1 into a router. There are several packages that allow you to generate iptables scripts which will establish this functionality. Some are GUI based, such as shorewall and many others. Others are canned, configurable packages scripts that do the same thing. My personal preference in this category is HomeLANSecurity, which I have used for many years with great success.
Since you are dealing with a laptop, there are probably implications for such a system, since I presume that you will run the laptop in other environments, where the router functionality would be inappropriate. I will leave it to others to comment on how you might deal with that. It probably wouldn't be difficult to simply run the iptables firewall script, or not, depending on where you are running the laptop.
The functionality you require is done routinely using this kind of method.
--- rod.