My understanding is that if an o/u shotgun was modified to fire two shots at the same time, it would be classified as a machinegun.
Yes but there are already double barrled shotguns made with double set triggers that work in which the forward trigger fires one barrel and the rear both(usually sxs). The propper way of firing such a gun is to pull the front trigger first and the rear second. If the rear is pulled first both barrels are fired. Its just the way they used to be made. Even though more than one cartrige is discharged the gun only fired once.
One function of the trigger, one shot, just two cartridges.