bwananelson:
I think you may be over-engineering the problem. While many owners of scoped doubles will advocate sighting the scope for one barrel, this is just silly IMHO, and entirely defeats the purpose of owning and shooting a double rifle.

Sight it for the centre of the composite group, just as if you were sighting it at the centre of a bolt-rifle group. Only difference is, the group will be a little larger.

Sight it at the distance you prefer for the cartridge's balllistics and the likely hunting ranges, just as if it were a bolt-rifle.

Simple really.