I believe they're referred to as a "swivel breech" design...essentially a double barrel firearm that only uses one hammer.
When I first learned of them my initial thought was it might be nice for a second shot at the ready...but finally decided:
1) A speed loader allows very fast reloading, in about the same time (or less) than it would take to do all that with the swivel breech;
2) A fast second shot is really not necessary in muzzleloader hunting;
3) They are a very heavy rifle with the extra barrel;
4) They don't have the typical lines of a muzzleloader rifle;
5) If they were really viewed as having a lot of positive attributes, everybody would be using them;
My .02 cents...