Multiple headphones on one computer
I haven't tried this, but if you're willing to experiment:

Try using (no joke) Garageband with your aggregated device setup.  No need to hit the record button, but you can use the monitor function to hear yourself speak.  And since it presumably knows what to do with both mics, you might both be able to speak.

You may also use LineIn to be able to hear yourself speak.  And finally, soundflower may be useful if you need to shuttle audio from one app to another before making it to your headphones... like say background music from iTunes or whatever.

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Update from OP:


  Thanks, I ended up going with something based on the ideas in your
  post but developed the solution on my own. Here it is:
  pivotallabs.com/ultimate-headphone-pairing