To understand why we hold our weapons with two hands, vs the original intent of the handgun, you have to look way back to the design of the first successfull handguns. They weren't as much a handgun as they were a very short rifle. They were the first true "point and click" interface. The barrels were mostly long and when you stuck one out at the end of your arm, it was something akin to pointing a short cane. One didn't need considerable accuracy to "considerably accurately" place the ball center mass. Formal training was nonexistent. People with moderately good hand/eye coordination could shoot reasonabley well. With the advent of the percussion revolver, the guns were still of long barrel, the grips not so different from those of the muzzleloading single shots and the line of sight still straight down the barrel. Shooting them was an instinctive exercise just as it was in days of yore.
Hickock was able to shoot his Navy pistols well, because everyone who has ever shot a Navy pistol will be able to tell you, they point like lasers! They have terrible sights, but one can nearly close their eyes and strike a target with reasonable accuracy. I shoot my modern handguns two handed, but almost always shoot my cap and ball handguns one handed when headshooting squirrels at reasonable ranges of 10 yards or less. If They are farther than 10 yards...I let them go. If they were ner-do-wells...I'd shoot a mite farther.
Dan