Yep, non-obvious but completely doable:
1. Plug in the two USB mics.
2. In the Applications > Utilities, launch Audio/MIDI Setup.
3. In the Audio menu of the Utility, select Open Aggregate Device Editor.
4. Click the plus sign to create an aggregate device.
5. In the lower box, select both USB mic AND the Built-In Audio.
6. Use the clock from the Built-In Audio, and check Resample next to both USB mics. This will synchronize all the clocks using the one associated with the internal audio.
7. Call the aggregate device whatever you want.
8. Now go to your System Preferences and select Sound.
9. Set the levels for each mic separately -- you can't set the levels using the aggregate.
10. Finally, in GarageBand, create a track for each mic, and select any Mono input (in the Track Info Details section) that happens to work.
I've occasionally had trouble where the signal gets static. Just make some changes in GarageBand, or unplug the mics to make it go away.
