So yesterday I checked through the Line 6 online forums and found a few interesting things.
1) someone asked if you can record multiple tracks at the same time in GarageBand, to which another person helpfully replied; Yes.
2) Other users recommended Tracktion from Mackie
I also discovered that there was a software update for the TonePort's software.
Armed with this information, I set about trying to get this all figured out last night.
The first thing I tried was updating the UX2's software. I had to register to download the software, and I was having network problems so it took a looong time to download the software. I mean a looong time. And then after I installed the software I discovered that it didn't solve the problem. Deck still didn't recognize the TonePort at all.
Then I tried Tracktion. Took a bit of figuring to get it to work with the TonePort, and I wasn't exactly thrilled with the interface. Also, the demo version plays an audible noise every few seconds just to be super annoying. I gave up after a while.
I then went to take a look at Bias's support, on the chance that maybe they had something about using the TonePort (nope) or that their forum might have something.
I had to find my product serial number to get into the forum. And when I did, I discovered a notice about there being a problem with Deck and Tiger (which I had to upgrade to when I installed the TonePort's software *sigh*) The notice said you could work around it by creating an Aggregate Device through Audio MIDI Setup. Insert a couple of hours of mucking about here. Long story short; discovered that the reason it wasn't working was that Deck doesn't support 24bit devices (i.e. TonePort.)
Then it occurred to me that I was using GarageBand 1.0 on the Mac, and I had a copy of GarageBand 2.0.
Sure enough, installed GarageBand 2.0 and discovered that it added a record button to each track; ta da!!! Now you can record to two tracks at once.
What did we learn from this? Not much other than the fact that what could have been solved in 15 minutes actually took me more than half a day.