is there any advantage to taking the crystal's output from the 8u2's PC7 pin, rather than directly from the crystal output pin?
I would think so, that's what it's designed for AFAIK. The normal 328 data sheet calls it a "buffered clock output", so it should drive heaps of stuff. I don't have an 8u2 data sheet but see no reason they would be different in this respect.
My first guess would be that the output wouldn't be enough to drive two clock inputs at once
It would only be driving one wouldn't it? The main processor. And anyway as I said, it's supposed to be buffered.
you couldn't just connect XTAL1 outputs to the same terminal of the same crystal.
I wouldn't bet on that working but it might. The output of an oscillator is normally just the output of an inverter, however unlike normal inverters it's probably not designed to drive extra stuff but it still might work. Even if it did I think though that this would be a flaky design.
Rob