I shoot with a black powder percussion muzzle loader exclusively (due to a brief lapse in judgement many years ago), so I have to deal with cocking hammers on grouse (and everything else). Personally, I can't double cock a gun in the field anything like quick enough to shoot a chukar, much less a grouse. I have had pretty considerable success hunting with my right thumb on the hammer at half cock, and just cocking that barrel when mounting the gun. Since I have never made a meaningful (i.e. close) second shot on a grouse even with a cartridge gun, I've concluded that it's not limiting me much. When I miss, I can switch to the left barrel to try for a second flush. I suspect that cocking both hammers at once is more a driven shoot technique.
