I'm not saying it's a good idea, or anything.
But, about 30 years ago I loaded 90 gr. of FFG black powder behind two .490 round balls. I patched both balls individually, packed them down over the powder with the bullets next to each other, and blasted away into a nearby tree stump. It was something I read about some hunter doing way back when he was hunting brown bears in Alaska. I dug the balls out of the stump and they were fused together in one big blob of lead.
The rifle was a .50 caliber Thompson-Center Hawkens. While I'm in self-disclosure mode, I also shot a ramrod out of the same rifle (no bullet) just like Brian Kieth did in the move "Mountain Men". Unlike the bullets, I never found the ramrod.
Probably, a maxi ball has more lead than two round balls, so there's no real good reason to double load a smokepole.
Ah, the perils of a misspent youth; ten feet tall, bullet proof, and fire-resistant.