If you’re playing as Leon Kennedy in Resident Evil 6, then you can dual wield his pistols, which are named the Wing Shooters.
Sorry guys but Leon is the only character that can Duel Wield.
To dual wield Leon’s pistols, just equip the pistols and hold the aim button, then press switch weapon button (Y on 360, Triangle on PS3).
Alternatively there is an option in the drop down menu when you select the Wing Shooters in the weapons menu to toggle dual wield.
To switch back to one Wing Shooter, just do the process again.
If you’re out of ammo with the Wing Shooters, you can actually switch to dual wield mode to magically get a little bit of ammo back.