The double hammers on each side allow for two loads per barrel.
The first load is dropped and the patched ball seated, then another load is dropped over that ball and another ball seated.
Front hammers capped and fired first of course.
I imagine that a very precise load and ball OAL is critical as to not get in the way of the flash hole for the front charge.
I've seen this arrangement before on another rifle - forgot where.
The rear load must get quite compressed from firing the front load!