How can i aggregate two pairs of headphones/microphones into one audio stream using Audio MIDI Setup?

I am trying to use the Audio MIDI Setup program to aggregate two USB Sennheiser PC8 stereo headsets so I can use both simultaneously on Skype calls and GarageBand recordings. So far I am only able to use one at a time by deselecting the other USB device under the newly created aggregate device. How can I get both two work at the same time!?

Thanks!

• Mac OS X Yosemite 10.10.2


• 13in Macbook Air Mid-2012


• Sennheiser USB PC8 Headsets (http://ift.tt/Zpx83q)

Screen Grab of Audio MIDI Setup