Oh, done it more than a few times, stretching a double’s legs. I hunted with doubles for a decade, and guided here in BC with them. They’re just two rifles soldered together and shoot long as well as anything, here’s a gemsbok I took in the Kalahari. We’re not discussing hitting at longer than typical range with a double, we’re talking regulation which is each barrel and where it shoots relative to the other at different distances. You can shoot far with double just as easily as a bolt action if you sight to the best barrel. You cannot expect the other barrel to land in the same area code however once well past the regulation point. That’s a factor fixed on the work bench in solder.
Doubles converge, then pass each other at some point in space. We all picture the lateral convergence, but there’s also a vertical factor as soldered barrels and harmonics contain a bit of voodoo. Typically this is between 50 and 100 yards. As you get to 150+, they’ll often be quite far apart and separating rapidly,
especially with an old .470. They were typically regulated ‘danger close’ due to their job description, and with loads that aren’t easily replicated today. Expecting to have a century old .470 shoot parallel and tight to 150 yards is unfortunately a recipe for disappointment.
Hunted that Gemsbok around the time you left. 
