Drag,
It's the one shot per one trigger pull issue, not the firing of two barrels at once using two triggers.
The feds could not care less if you fire both barrels at the same time in a double shotgun, or double rifle, or howdah pistol, as long as you use TWO triggers to do it.
If a single trigger fires more than one shot with a single trigger pull, it's a "machinegun".
Yes, there are loads of dual-triggered doubles out there, but each only fires one shot with one trigger.
Even the single-triggered models will automatically choose one barrel or the other, won't fire both barrels with one trigger pull.
The definition wasn't meant for shotguns, or double-barreled anythings, it was meant for full-auto guns & the broad definition prohibits what the OP is talking about by technical application as a "by-product". Or collateral damage, if you want to use the term loosely.
Denis