T1 ToneMatch® Audio Engine / Recording / GarageBand

Preliminary

These notes originally appeared in T1 as part of OSX Aggregate Audio Device with thanks to DaveNJ

1. Open Audio Midi Setup and ensure that the interfaces you want to "combine" are set for the same bit depth/resolution (in this example, Built-in Input and Bose USB - both should be at 48000 and 24bit):

2. Then, choose "Open Aggregate Device Editor" from the Audio menu:

3. Next, click the "+" button to add a new device and check the "Use" checkbox next to Bose USB Audio and Built-in Input - you'll see the number of inputs add up to 4 (in this case):

You can name the device whatever you want...

4. Save it, and then select it in GarageBand:

That should do it...