Dave,
I was on the receiving end of both barrels of an old SxS 10 gauge going off at the same time. I'm still kind of hazy on what happened (hell, I'm hazy about the whole frigging day), but I either tripped both triggers at the same time somehow, or the recoil from the first barrel dislodged the hammer on the second one.
All I know is that there was this HUGE ringing in my ears, new and unexplored universes of stars circling my head, and blood running out of my nose where my wrist, or the shotgun's wrist, had smashed into it.
Of course, my friends, being the concerned 20somethings that we all were back then, were busy staining their shorts yellow they were laughing so hard.
Of course, none of them were brave enough (or stupid enough) to load both barrels after that.