Jim, since you mentioned a double flinter I'll tell you about the one my friend MADE. He used to work for a company called Holland & Holland and one day he came across a pair of scrap s/s barrels. There was nothing structurally wrong with them they were just not up to the usual standard. I seem to remember the rib was not laid quite square. Anyway he was given them for his project.
He made breech blocks and then set about making a single flintlock. He had a workmate who was a stocker who undertook the woodwork. The eventual gun was made as an o/u rather than s/s. Now to the clever bit. He made the pan quite deep and put a spring loaded divider in which was released when the cock went to it's half cock position. So, to load, you stuffed both barrels as one does with a front stuffer. Then put the lock into half cock(divider opens automatically), powder bottom of pan, press divider closed and fill top half of pan.
When ready to fire, fully cock piece, fire top barrel. All you had to do then was to cock again (as cock went thru 1/2 way, pan divider opens automatically exposing primer for bottom barrel) close frizzen and fire again.
The result was a thing of beauty which was engraved "Made by ***** ***** and ***** ***** at Holland & Holland (Date)". He once offered to sell it to me but at the time I wanted a .45. WRONG DECISION.