1. Go to Applications > Utilities > Audio MIDI Setup on your Mac and open the Audio Devices window. You will see all your devices in the list on the left hand side.
2. Click on the PLUS button (+) at the bottom of the Audio Devices window and choose Create Mulyi-output Device / Create Aggregate Device.
3. Select the devices you wish to include in the Aggregate Device by ticking the corresponding box in the Use column. You can rearrange what order the devices are in by dragging them to the spot you want.