I did some research regarding this matter and found absolutely nothing anywhere that even mentions using multiple network adapters under BeOS.
It was my initial assumption that BeOS networking does not natively or normally support this. BeOS was intended as a home computing desktop OS with emphasis on multimedia, so it's doubtful that developers would have considered supporting multiple network adapters.
Please note that I do not actually *know* that this is true, but I seriously doubt BeOS's network support will let you use multiple network adapters simultaneously. I just don't see any way this could work.
However, thanks to the modular design of BeOS, network support (the net server) isn't built into the kernel; it's just a module. This means that it's possible to code around this, although I don't much see the point.
Honestly, I think your best bet may be to use a single network adapter. I'm certainly no expert on network topology, but surely you can access both networks using a single NIC.
I know this probably wasn't the sort of answer you were looking for, but a simple solution may not be available.
As far as the Devices Preferences, all sorts of things you can't actually use can show up there. You don't even have to have a driver loaded for a device to be listed. For example, my Midiman/M-Audio Delta 1010 shows up in Devices, but thanks to Midiman dropping the ball on the BeOS they PROMISED in a press release years ago and which were always "just around the corner", there's no driver and no way for me to use it. It still shows up, though. You can also add anything you like to the list of Devices.