Dual External Mic Aggregate Not Working

When recording on a macbook air, you must go to the Audio/Midi to create an aggregate to use more than one external mic for input devices at once. I select all necessary devices and continue to try to test my mics (2x Audio Technica ATR2100). When I go to System Preferences>Sound>Input, and test each mic, the volume display shows that each mic works. I tested this by turning off each mic and testing the other to make sure each was working on their own on the aggregate selection. However, when I select this aggregate on GarageBand or other media, only one of the mics works. Anyone know why this may happen? Thanks.