I do not have the strength or the dexterity to mount my hammer gun when a bird flushes and to simultaneously pull back both hammers. If I am alone I hunt with both hammers cocked, always keeping the gun pointed away from the pups. I know this is an unsafe method. When hunting with friends I have kept the barrels open slightly with both hammers cocked. I can then quickly close the barrels while mounting the gun if a bird is moved. This method can be problematic, however, when hunting in heavy cover. In short, I really don't have a good and safe way to efficiently hunt upland birds afield. As a result, I rarely bring my old London, "Best" hammer gun afield these days, and that is a shame.
I wonder if some of the cocking difficulties could be in technique rather than strength and dexterity.
When I carry a double for pheasants, my thumb is over the hammers and my entire hand is probably rotated slightly towards the top of the gun, and the barrels are held higher than the wrist. Cocking happens while the barrels are still fairly high relative to the wrist and is almost the first part of the mounting process. My thumb cocks the left side but it is as much the web between thumb and first finger that is cocking the right barrel at the same time. As the barrels come down and butt comes up, the gun is cocked, and the right hand rotates slightly to the shooting position. It is very quick and simple. But if I mounted first, then cocked, I think it would be quite a bit more difficult.
While some guns are easier than others, it does not take much strength or dexterity to do this with any of them, which is why I think that it may be technique that is the issue.
Maybe this will help someone.