Have you tried it?
I'm not a MIDI Expert, but as I understood it, GB listens on all MIDI channels, so if you can connect more than 1 device to your computer, GB should use them (all on one track, of course, as you're apparently aware).
If you have tried it and it didn't work, hang tight, we have several folks here very well versed in MIDI (2 are overseas, so they might be in the middle of late dinners, or something)