FWIW, I've see double muzzle-loaders that were regulated by filing the muzzles shorter on the outside than on the rib side. The theory as explained to me was that releasing pressure on the side opposite would cause the projectiles to cross at a greater distance.
It sounds kind of radical and I would consider trying it on an old "beater" double before hacking up a good gun.
The couple that I saw were filed back about 3/32" on the side.