To get the same amount of ammo as you would get for dual wielding two of the same gun, have one of the desired gun in your secondary while having a dual wieldable gun as your primary. Dual wield a different gun in your left hand, then press RB to swap the right-hand gun for the same gun as your secondary. If done correctly, you will have the maximum ammo as if you were dual wielding the gun in your right hand. You can also get even more ammo by dual wielding the same gun. Note: This does not work for battery guns. Also, you must keep the same gun in both slots or you will lose the extra ammo.