Parallel trajectories.

As distance opens the group size, barrel spacing makes less of a difference.
Even at twenty yards the distance of a group's center from the sight line is irrelevant - the double rifle is a hunting rifle after all.

Furthermore there is no good reason to settle for poor accuracy around the build and accuracy of a double rifle. We've seen time and time again excellent field accuracy from these rifles, even from 150 year old guns.

And the notion that a double rifle is only a dangerous game gun or only a close quarters gun or only a gun for stopping charges etc is hogwash. We've seen front-loading double rifles in small calibers from early on which were obviously built for deer or smaller game, as well as cartridge double rifles in pea-size calibers for rook and small game through time to modern times with examples in rimfire and pistol cartridges and high intensity small bore cartridges for deer etc.