Have two original combination rifle/shotguns - both over/under with rifle sights. If course, the rifle sights are for the rifle barrel, so it's not the same as two smoothbores. That said, just as mentioned earlier, regulating two barrels to shoot in the same place at varying distances is, from my research, difficult.
One of mine is a .54/12 gauge which I tried with modern 12 gauge slugs on a whim. Even at 25 yards, they weren't even close. At 50 yards, I might as well have been throwing rocks with the 12 gauge barrel. Patched round ball may have done better, don't know. Didn't have any.