Dual Output via USB on Mac OS X
This tutorial will show you how to change the settings on your Mac so it will output sound to two USB audio devices at the same time.
Open the spotlight menu by pressing Command and the Space Bar
Type “midi” and then click on “Audio MIDI Setup”
Click “+” and select “Create Aggregate Device”
Check the boxes next to the headsets
Click the triangle to the left of the Aggregate Device to expand the menu
Click on the first headset, make sure it is set to “1ch-16bit Integer” and that the Volume slider is to the right
Repeat this step for the second device!
Make sure that you repeated the previous step for each device.
Seriously, it might not work if you skipped it.