Sure, I know how to do some of the hookup (no knowledge of GarageBand or iPads, sorry).
I know what the TEC BC 2 is sending. What exactly will the midified vibrandoneon send? Does it have velocity sensing on the notes?
I’m going to guess that there will be no velocity sensing. Since MIDI requires that notes have both pitch and velocity information, I’ll guess that the controller will send a fixed velocity.
GarageBand looks like it supports MIDI instruments as plug-ins. It doesn’t matter how it does it, what matters is whether the instrument supports breath control. If the instrument supports breath control, then route both controllers to the instrument. On my DAW, I would just set up a track so that it inputs from all MIDI controllers. The two controllers will need to send on the same channel (usually 1).
If the instrument doesn’t support breath control, then it probably supports volume. This is not the same thing at all, but for some simple MIDI instruments, it doesn’t matter as they are not very sophisticated. You have to somehow map CC 2 (breath control) to CC 7 (volume). This is a GarageBand thing, and I’m sure a web search will tell you how.
Usually breath control values change the timbre of the sound; soft notes don’t sound the same as loud notes. Volume control doesn’t–it’s just volume. You get one timbre; it’s quiet or loud, but it’s the same sound. It’s OK if you’re not really trying to emulate a real instrument.
If you simply mount the TEC BC 2 on the pipe, that’s fine. You’re going to have the exhaust sound right next to your ear, though. Your mileage might vary, but I’ve found it very obnoxious. It’s OK on quiet passages, but not loud ones.