I have to agree with Hanshi...
From my observations, if you are reloading..., you're not getting a "quick" second shot..., and also if you are in that much of a hurry for your first shot wasn't good enough when you took the time, then how effectively placed will your second "quick"shot be?
I know of three ways to get a "quick" second shot...,
I have a .50 caliber caplock swivel breeched rifle, and I can re-cock and swivel to the second barrel in about 3 seconds..., and a SxS rifle with both hammers cocked could be even faster (assuming the cloud of smoke from the first round doesn't obscure the deer). Lastly, one could carry more than one loaded rifle (or I suppose one could "Hawkeye" the situation and have a gun-boy pass you a second, loaded rifle. :haha: )
Other than that.... nope.
In 20 years of successful deer hunting..., I have had two opportunites for such a shot... the first was a flash-in-the-pan, and had I been using the swivel breech or a double gun, I could have fired the second shot to harvest the deer. The other time I took one of nine does from a group, and for some reason of the remaining 8 animals, 7 fled from my shot, but one stood still staring in my direction, and I could have taken a second deer had I another barrel.
It's not really a factor (imho) and I'd suggest placing a good first shot, and waiting 20 minutes to get the animal. Actually..., if you are quiet, and you reload in about 30 seconds and then sit for the rest of the time..., another deer might wander by giving you a chance at a second deer, instead of worrying about shooting the first deer twice.
LD