It is preferred to use one sight for both barrels for hunting situations so you don't have to move to make another shot, or take time, or make mistakes while fumbling with the second sight. What a cool thing it is to have a quick follow-up shot with a musket in case your first shot clips a twig and the deer is still standing there. If you move the deer will likely run away. The second sight on these rifles could be used for a different range (for both barrels).
I don't know why double barrel smoothbores or rifles were not used more as a historical battle weapon. Cost perhaps?
