When I started into muzzleloading, in 1980, the only info available was the Dixie catalog. In there, they suggested that you proof test your new Dixie muzzleloader, with an overcharge of powder [I can't remember how much], and two balls.
So, I did this with my new .50 TC Hawken. I can't recall, might have been 100 grains of powder and 2 balls, each with its own patch.
As Dixie recommended, I braced the gun in a tire, a pulled the trigger with a 50 foot string.
No problem.
Well, I got into deer hunting, and had killed a few deer with this gun. I got the idea of shooting two balls at the deer. I figured it would hit harder. Not that I had any trouble killing a deer with a single ball; in fact; the .490 round ball was a great deer killer. I was just experimenting.
I was using 80 grains of powder, and two balls. At the range, it was surprisingly accurate. The two balls would hit about 2 or 3 inches apart, on a 50 yard shot. It was still hitting close to the point of aim.
I figured it would be pretty effective on a 50 yard shot.
I tried a few shots at 100, the balls were pretty far apart at that range.
I never took this load into the woods, as I did not want to try a 100 yard shot with this load.
The .50 round ball weighs about 174 grains. If your rifle is designed to handle a 350 grain slug, there is no reason you couldn't shoot two round balls. If you try it, do not increase the powder. Generally with a big lead conical, you will reduce the powder.
As for the idea of taking a brush shot, because one ball might hit a tree, I don't like it. If one ball might hit the deer, and one might hit a tree, it is just as likely that one ball will hit a tree, and the other ball will hit the guts.