In general, yes, but mainly depends on what job each one is supposed to do. One of the common stock configs on the Xserve G4 was dual ethernet - the integrated one and a PCI card in the single-slot riser. (In fact, the only way to take advantage of the second ethernet activity LED on the front panel is to use a supported PCI card in the single-slot riser.) I gather Apple intended this setup to perform router/gateway functions.
If you're talking specifically OS 9 and earlier, I believe AppleTalk can only be assigned to one device at a time with stock software. If you get some more sophisticated router software involved then you should be able to bridge the connections in some manner. Connecting them both to the same network is a recipe for trouble. "Dual-attach" networking is a different beast.