I don't know much about regulating doubles although I have spent a lot of time shooting pistols, rifles and shotguns over a chronograph so know a bit about grouping and patterning.

Have you tried grouping from each individual barrel allowing time for cooling between shots. If one barrel shoots acceptable groups and the other does not then obviously you cannot expect to get good regulation.

If I were reloading for your double that is what I would try first, to get a load that groups well in one barrel then try the other. A load that groups individually well in both barrels will have a better chance of regulating.