At risk of derailing....the dbl barreled muzzle loading shotguns are/were just a poor transition from early fowlers/fusils/trade guns and modern cartridge shotguns.
Each muzzle loading shotgun manufacturer tried different bores, started evolving "chokes"......the much older fowlers, fusils, trade guns were made basically the same.....to use shot or ball interchangeably. This required a straight bore.
Again this is just my theory but I think ML dbl barrel shotguns were designed for a particular load. The earlier guns were just accidentally better at giving different pattern with different loads. So think of SxS ML shotguns as "modern" when it comes to loading from the muzzle. Just transitional tools, not better because the only improvement was a faster second shot.
Added more.