From WikiGTA - The Complete Grand Theft Auto Walkthrough
The dual pistol (or twin pistols in Chinatown Wars) is a set of two normal pistols. It is possible to have both the dual pistol as a normal pistol in your arsenal, as they are seen as separate weapons. The dual pistol has a higher fire rate than the normal pistol. In GTA2, the dual pistol fires two bullets in a V-shape, which makes aiming harder than most weapons. The twin pistols in GTA Chinatown Wars fire straight ahead.
It is also possible to wield two pistols in GTA San Andreas when your weapon skill reaches a certain level, but this is simply an upgrade of the normal pistol in stead of a separate weapon.