In GTA 2, the "Dual pistol" is in fact separate firearms from the single pistol (with their own separate ammunition), giving the player the ability to fire bullets in a V shape formation and twice as fast and efficiently as a single pistol, but with the drawback of faster depletion of ammunition.
The ability was since been refined in GTA: San Andreas after being absent in GTA III and GTA Vice City. For San Andreas, the player will wield two pistols, "akimbo" style, once they reach the Hitman skill level for the pistol, but will use the same ammunition as a single pistol.
In GTA Chinatown Wars, the player has the option of using either a single pistol or "Dual pistols", both set of weapons sharing the same ammunition, the latter effectively better in speed, damage, and magazine size. They can be bought of the Ammu-Nation website for $450.