There is also one other option, probably most useful on scoped doubles of 9,3 and under.

If you will be hunting in an area where long shots will be the norm, and your left and right bbl shots hit farther from each other than you would like you could sight in your scope for the most accurate bbl at a distance say 200 to 250 yards.
Up close ie out to 150 yards your right and left bbl groups should still be close enough to hit a game animal, but if faced with a long shot you know you the one bbl will hit point of aim.

In fact if you have a good scope you should do this at the range so in the field you can adjust for the longer shots if necessary.This adds to the huntability of a double rifle, and can give you a little more comfidence if making a longer range shot.