I had fairly good results years ago shooting double balls out of a rifled .58, that had slow and shallow minnie rifling. They still held together well out to about 70 yards as I remember.
I should think that it would work well in a smooth bore. I aways thought that putting a half of a shotgun wad between the balls might reduce deformation and improve accuracy. I've never tried it in my Brown Bess, because the ball is so damn big that I don't think two will make much difference! If something gets hit by one 585 grain ball, it's gonna be a train wreck! Also, two balls would be well over 1000 grains of lead, which might raise some pretty high pressures. :shock:
Does that barrel have front and rear sights? With rifle sights, and a tightly patched ball, a smooth bore will really shoot. I have traditional full buckhorn rifle sights on my Bess, and she'll eaisly stay on a paper plate at 85 yards, and it will shoot 3" groups at 50. By the way, in a smoothbore, seat the ball with the sprue DOWN.
If you experiment with this be very sure that both balls are touching each other, or seated one on the other, for obvious reasons.