I doubt the character tables could hold Bebop and Rocksteady at the same time. However, in speaking in the hypothetical, you'd be better off with B being melee attack, double-tap being dash attack, A being jump and A+B being "special," the gun. The gun would need some type of limit, could stick with old standards of it costing a drop in your health, or simply an ammo meter with ammo powerups being scarce.