Good post.
In 1860, Leut. James Forsyth & posted to India, wrote the book The Sporting Rifle and It's Projectiles. I this book, he stated the barrels should shoot parallel, that being that the centres of each barrel's groups should be the distance apart, the same as the bore axis' apart so the sights can be trusted at any range.
This makes total sense.
On the other hand, a rifle such as a SxS muzzleloading rifle that will likely not be used past 100 yards, could indeed print one barrel on top of the other at for instance, 50yards, so that at 100yards, they would likely cross by the distance the barrels are apart - no big deal as the total group of both barrels would be the same as if they shot parallel. At further distances of course, those barrels would continue to diverge. A good shot could easily make allowances, however - allowances would have to be made. If the barrels shot parallel, no allowance would have to be made.