It’s not. It’s a Hollywood thing, because it looks cool. There were reasons for dual wielding firearms historically, but those don’t apply with modern pistols.
With early single shot pistols, rotating through weapons rather than reloading them was a (rarely used) option. This would lead to shooters firing one pistol, putting it away and selecting another, while firing with a pistol in their other hand. The most famous practitioner is probably Blackbeard (Edward Teach), who would go into combat with six pistols on a bandoleer.
With some 19th century revolvers, like the Colt Walker and Colt Single Action Army, the guns are balanced for use in a single hand, to the point that it’s actually uncomfortable to use in a modern stance. With these it is possible to accurately fire with one hand, and then switch to a revolver in the other hand. With practice it may be possible to line up a shot and fire one while recocking the other, but this is more of a trick shooting tactic than something someone would use in combat.
It’s probably also worth pointing out that fanning the hammer is also a trick shooting technique, and not something that you’d use in combat. In general, bullets were too expensive to waste that way.
All of that said, it can be a fun way to vary up your shooting on the range. So you will occasionally see people who practice it. But, it’s not something to take into a real firefight.