So, bottom line is, it's most likely illegal to connect the triggers to fire together.
So ... what if the only thing connecting the triggers is a chubby trigger finger which itself is able to automagically fire both barrels simultaneously?
Would that be simply like shooting two guns at once?
Ah well ... I just found the picture funny and started thinking out loud. I'm not gonna waste money on 2 shotguns to make an infernal device like that.