How can i aggregate two pairs of headphones/microphones into one audio stream using Audio MIDI Setup?
Not sure it's going to work using an Aggregate Device - each output must have its own channel that way.

Look at a Multi-Output Device instead - then channels can share one output.

You of course, don't need to use the same 'device' for both input &amp; output, so you can set up a different one for input - though I'm still not certain you will be able to listen to [record] both mics at the same time, the same considerations may apply.

I can't test on USB headphones as I don't have any, but using this setup I can play iTunes over internal speakers and my dedicated sound card at the same time
sounds horrible, of course;)