A ridiculous, idiotic, and downright fucktarded practice popularized in John Woo movies and unrealistic video games like Call of Duty.
It is probably descended from the few gunhands in the old West who used two pistols. But instead of firing both at once, they would either do a Border Switch (draw two guns, empty the first one before firing the second), or a New York Reload (draw your backup piece when your weapon runs dry rather than reloading). This is because it is impossible for a human being to look through two sets of sights at once.
Therefore if you fire two guns at once, at least one of them won't be aimed at all, causing you to waste all of the ammo in that gun hitting nothing that you actually wanted to. Or you could just do it the Hollywood way and not even try to use your sights, meaning you'll just spray & pray with both weapons, waste all of your ammo, and hit everything except your target. Also, using two SMGs or automatic rifles instead of pistols doesn't make this stupidity more effective, it just makes you waste more ammo, look more retarded, and still not hit jack shit.
2) To use two one-handed melee weapons at the same time, rather than one weapon and a shield. Seen a lot in all kinds of RPGs. Probably would be effective in real life if mastered, but seems like no one ever bothered to try.
3) To stick one fist into the girl's baby hole and the other fist into her poohole.
4) To jerk off two cocks at the same time, wielding them akin to clubs or swords. A display of great multitasking, passion and ingenuity.
The practice has its start in the Old West. when cowboys generally carried single-action revolvers. Since these weapons generally had no more than a six round capacity and slow reload times, it was often necessary to carry more than one handgun. However, since most of the handguns were single-action, it was very impractical to dual-wield them. It is more likely that the cowboys used what is called a New York Reload which is to hold one gun in each hand, firing with the strong hand (generally the right) and then, when that gun ran out of ammunition, switching it out with the gun in the weak hand.
Dual-wielding as it is known today first became common in kung fu movies featuring guns, such as those directed by John Woo (called gun fu). The use in films later spread to some action and western films. Today, in addition to action movies and show, it is not uncommon to see akimbo style in video games.