the 28 ga brennecke is to light with only 273 grains. I would go for the 20 ga mag with 473 grains, this is the force.
years ago I see a pair of howdah pistols, one in 577 snider and the other 20ga smooth with roundballs.
sawed off double guns are a legal problem in the US if I understand it correctly. you have to pay 200 bucks to the ATF for register the gun to make it legal?
here it would cost similar including a visit to the proof house to proof it again ( any real change one the barrels need a new proof) but then it would be a "pistol" and need one of the common two pistol licences you get.