The OP introduced the thread with a couple of very interesting pictures of a double-barreled flintlock fowler he found for sale on the web. Lots of interesting comments followed re. this unusual firearm, especially the mod that was done to it.
Below are two photos I copied from the selling website. The firearm description is an Antique English/American Double Barrel Flintlock Fowler, by 'SHARPE, LONDON', circa. 1780-1810. The selling price is $4250.00. (Could this be a bargain price since the buyer is getting two BP firing systems?!?).

Photos are simply for your viewing entertainment. Enjoy.

PT7
Right-side barrel uses a percussion lock.
Left-side barrel uses a flintlock.