Pretty sure Pedersoli mount the bores parallel and hopes for the best. I got the .58 I had to shoot into the same small group at 50 yards using round balls, thus it shot fine to 100. With slugs, however, it shot a foot apart at 50 & no load brought them closer. In other words, it was useless with slugs.

According to Forsyth, some English makers prior to 1860, thought regulation meant building the barrels in parallel.
His contention was that they should be regulated by adjusting the dimensions that when joined, would show crossing, but in firing the recoil would then make them shoot parallel.
He noted that with a gun that did not regulate, that is to shoot parallel, to sight the rifle for the most accurate barrel, and at least have one barrel to trust, learning to hold off for the other, poor barrel.
At one time I wanted to rifle the very tight chokes of a SxS .410 shotgun, but did not have "one" to lay with and my buddy did not want to offer his as it would then to TOTALLY useless for potting birds.
I thought with light cast pistol bullets in the 200gr. range and using 3" hulls & black powder, it would make a dandy deer rifle, about like a .40/82 Winchester, or 40/90 Sharps, but with lighter bullets. I did not even get around to finding what a hull would hold, but likely close to 90gr.