Howdy
First off, let's be clear about this: Although some historically minded outsiders look down their noses, Cowboy Action Shooting (CAS) always involves shooting two revolvers.
There are basically two ways to shoot a revolver in Cowboy Action Shooting.
One way is to shoot the revolver one handed, cocking the hammer with the thumb of that hand and pulling the trigger with the trigger finger of that hand. This is known in Cowboy Speak as Duelist Style. Touching the revolver with the other hand while shooting Duelist is not allowed, except to clear a malfunction. There are a couple of variations. Double Duelist shooters shoot one pistol with one hand, then shoot the second pistol with the other hand. Then there is Gunfighter, which is the only time two loaded pistols can be in the shooters hands at the same time. When shooting Gunfighter, the pistols are fired alternatively, first one, then the other, alternating both pistols until the course of fire is completed. Shooting Duelist generally stresses the pistol the least. The 'strong hand' thumb does not have enough strength to yank the hammer back hard enough to stress the lockwork of the pistol. Also, very generally speaking, Duelist shooters tend to be a little bit slower than shooters who hold their pistols with two hands. Also in the one handed categories is Frontier Cartridge Duelist. That is when the shooter shoots his pistol one handed, but the ammunition is loaded with Black Powder or a substitute. That is the category I always shoot in.
The fastest shooters in CAS hold their pistols with two hands. The pistol is held with one hand, the 'strong hand'. The trigger is pulled by the trigger finger of the 'strong hand' while the hammer is cocked by the thumb of the other hand. Note, this is not fanning. More about fanning in a moment. With a two handed hold on the pistol the off hand thumb is able to cock the hammer very quickly, faster than in duelist style, because the off hand thumb can rotate to cock the hammer very quickly. This is not physically possible when shooting one handed, the strong hand thumb does not flex that way. There are two basic variations on two handed shooting. In one variation, the off hand thumb cocks the hammer and the strong hand trigger finger pulls the trigger separately for every shot. Slip Hammering differs from regular two handed shooting in that the strong hand holds the trigger back for the entire shot string while the off hand thumb cocks and releases the hammer for every shot. Both of these techniques are very fast, and it is virtually impossible for an observer to tell if the shooter is Slip Hammering (holding the trigger back for the entire shot string) or pulling the trigger separately for each shot. Both techniques are legal.
Fanning is forbidden in CAS. Fanning involves shooting the pistol from the hip, without bringing it up to the eyes to sight it. The off hand slams the hammer back while the hand holding the pistol pulls the trigger. The reason fanning is forbidden in CAS is because it is not a controlled way to shoot a pistol. Shooting from the hip, the shooter cannot sight the pistol, and the action of slamming the hammer back with the other hand will tend to cause the pistol to move and point somewhere other than where it started pointing. Fast draw competitors usually fan their pistols, but they are never shooting live ammunition. They are either shooting blanks, or ammunition loaded with wax bullets. CAS ammunition is always live ammunition, loaded with lead bullets.
Of these techniques, fanning is the most likely to damage a pistol. The force imparted by the hand slamming the hammer back will be directly transmitted to the delicate internal lockwork parts of a traditional Colt style single action revolver. Fast draw competitors often shoot pistols that have been modified inside to put up with the stresses fanning causes to the lockwork.
Shooting a pistol two handed tends to put more stress on the lockwork of a single action revolver then shooting one handed because the off hand thumb can exert more force on the hammer than is possible when shooting one handed. Don't get me wrong, I have seldom seen a single action revolver actually damaged by a two handed shooter, but it does stress the lockwork more. The hammer slams back faster, and the cylinder revolves more quickly, putting more stress on the bolt as the momentum of the cylinder is broken when the bolt stops the cylinder from rotating.
As stated earlier, shooting a pistol one handed tends to stress the pistol the least because the strong hand thumb is not capable of yanking the hammer back as quickly as the off hand thumb can.
Finally, for all those who have been going apoplectic when I refer to our revolvers as pistols: In CAS we always refer to our revolvers as pistols. Just part of the game.