For the record, and anybody on the internet searching for this, if you want an Aggregate device consisting of your two USB microphones to be available (and even show up) for recording in Logic Pro 8 then you need to include an output into the aggregate setup, which I assume it's something to do with allowing monitoring. Garageband and Soundtrack Pro don't have this problem.
All is well in the world now, I can avoid a messy step of podcast production now and stay in nice Pro app land all the way.