You can record two tracks simultaneously with the UX2, so yes, you could record them both at the same time and have them as separate tracks in Garage Band. The one thing to consider, though, is that the drums would probably sound best if recorded in stereo. Really, the best way to record electronic drums would be to take a separate output from each drum and a stereo track for the cymbals. That way you could apply different effects to everything. But if it's just for a rough demo, a stereo track would do fine. Mono would be OK, but I think I'd try to do stereo.
The other thing you could do with electronic drums is to record a MIDI track from the drum set. You're right, the UX2 doesn't do that, but you could a simple MIDI to USB adapter and record the MIDI track. Than it would be a pretty simple thing to create an audio drum track with that MIDI track using virtual instruments in GarageBand. The cool thing about that is that you don't have to commit to a drum sound while recording the part.
Sorry if this is more information than you want or need. As with most recording things, you can get as simple or as complicated as you want.
Time is a train
Makes the future the past
Leaves you standing in the station
Your face pressed up against the glass