Oh WOW Im onboard for this one! I would say 6-8 inch barrels, maybe even a second set OR a duplicate gun with longer (say 14 inches) and a detachable shoulder stock , packaged as a set.

Yes I like the hammer idea too; but with them tilted just a bit into the center so that they can be cocked at the same time with one hand-Taylors sells a SxS Coach Gun like that. Id want a side lever over a top lever; but Im flexible on that one. Also a true pistol grip (like the one pictured).

Cartridge, Hmmmm? Lets see 45-70 would be the easy button but maybe something a bit shorter. Id certainly prefer bigger. How'bout 50-70? Theres always 500S&W if youre open to smokeless

Youll have to school me on why it has to have rifled barrels. I recall some talk awhile back about I seem to a cartridge based on the brass 20ga shells shortened to 2 inches called 600 Manchester. One version had round ball exposed but held in place with 5 or 6 crimps. The other had a conical loaded like a bullet. I dont recall any load data other than it was a BP load. Seems to me that would have the punch and look