There is an overemphasis on regulation IMHO. Firstly, a gun must be regulated for a specific load and conditions. A gun regulated for Nairobi won't necessarily be regulated for Alaska. secondly, almost invariably only one barrel is used, with the other held for backup - the latter almost invariable used at close range. I've examined quite a few older double guns that have one barrel significantly more worn than the other. Assuming a gun used for dangerous game where the first shot is going to be at something like 50 yards, the second will likely be at about half that, so the regulation only has to put the round into the target are at that range.
Double rifles are working guns, not range guns. They don't need to shoot MOA from both barrels. They need to work 100% of the time and be regulated enough to get the job done.
Finally, a lot of these rifles will never be used on a charging cape buffalo or elephant anyway, so what does it matter. if someone want to own a double for the fun of it, why not buy a less expensive gun (one in reach of more people) and spend some time working loads that will reasonably well.
My first double was a Navy arms 45-70 with external hammers bought some 20 years ago for a ludicrously low price. With a lot of experimentation I got it to shoot within 5 inches of POA at 50 yards with the right handload. Note that the right barrel is dead on target and hits within about 1.5 MOA of POA every time. I generally have no problem putting both rounds into a watermelon sized target at 50 yards. Probably good enough for its intended use. By comparison, my Merkel is only slightly better at about 10x the cost. It's a nicer gun of course, but as far as functionality?...