Here in Kentucky we are limited to a single projectile and I would think that would be true in most jurisdictions. I have heard some of the oldtimers talk about double balling at fun shoots. The idea was to break two ballons with a single pull of the trigger.
My concern with the buck and ball load, if you are using a s/s shotgun, would be the ball loosing contact with the powder which could cause a dangerous situation.
Sorry I can't be of more help.