I am not a attorney(thank god :)) but as I understand the law, it is ILLEGAL for a firearm to fire more than one round per operation of the trigger(unless NFA/class3).
I have heard that 2RB can be leagly acheved by using a device that fires 1 round on the PULLING of the trigger and one round on the RELESE of the trigger.
FWIW & YMMV,
Echo6