I have always thought of Cape guns as close in range meat collecting guns. Buck and bird.

Also good for defence of the laagered wagons.

One advantage of a cape gun. Just like a combination gun, proper regulation may not really be required. Sighting the sights in for the rifle barrel. If accurate enough, the single barrel might be accurate to whatever range a single shot might be accurate to. If the shotgun barrel is using shot, even large shot, rear sights might be mostly redundant.

If shooting ball or slugs in the shotgun barrel, of course the rear sights would be more useful if more aligned. Precise regulation would still not be needed or necessary.

So setting up the sights for the single rifle barrel would be the go. And then adjusting the shotgun smooth barrel to shoot at closer ranges close to it. IMO.