It depends on the species of game hunted. Duck hunting is a non-issue. There is usually time to cock both barrels while the birds are coming into range.
Upland game is a different issue. I have hunted many years with hammer guns, and have come on the system of cocking one barrel on the flush and then hold the second barrel in reserve for a second flush. Over the years of hunting grouse and woodcock, I can probably count on my fingers of situations where I would get two GOOD shots at one bird. More times than I care to admit, I have whiffed with two fleeting shots on one bird only to have a second bird flush moments later, only to be standing there with an empty gun. I have taken many birds with the reserved barrel strategy.
However, having said that, if you hunt in areas where multiple birds can flush at once (pheasants, quail, etc.) A hammer gun will limit your rate of fire.
This is for cartridge guns that generally have low hammers, and are difficult to cock with your thumb at the same time. Alternatively, percussion guns and other guns with high hammer spurs such as pinfires, can with practice, be cocked at the same time.
