The only two options i could think of that could create a sense of duel wielding at two targets is either to have a lock on feature like it was already suggested or to have a different kind of lock where it locks the position of your gun in relation to your body while you aim with the other gun.
This of course would block the amount of movement you could do with your camera since you shouldn't be able to turn more than 180 degrees from the way the locked gun is pointing.
Now how i imagine this could be pulled off is have the game react differently to how the two mouse buttons are pressed.
When neither one of the buttons is pressed, you are in your idle stance, you can rotate your camera anywhere you want and no bullets are being fired.
When you press both of the buttons at the same time you shoot both of your guns at the same time and follow the camera like in any other game with dual wield.
Similar thing happens when you press only one of the buttons where instead of both guns firing, only one fires.
Now here comes the tricky part: if while you are pressing one of the buttons(1), you press the other(2) one as well, the already firing gun(1) locks in place and the camera switches to the direction the now firing gun(2) is facing and you can aim with that gun(2).
While this is taking place, you can only rotate to a limited degree(just point in one direction with your left hand and while keeping your hand in that direction see how much you can rotate and point with your other hand).
To switch between hands while in this dual mode, just release the button of the gun(1) you wanna aim with and press it again(1). That way the previous gun(2) will be the locked one and the newly pressed one(1) will be the unlocked one.
Really confusing, i know.
But it works without the need for an ability and with enough practice can be used to hit multiple moving targets at the same time.
This of course would block the amount of movement you could do with your camera since you shouldn't be able to turn more than 180 degrees from the way the locked gun is pointing.
Now how i imagine this could be pulled off is have the game react differently to how the two mouse buttons are pressed.
When neither one of the buttons is pressed, you are in your idle stance, you can rotate your camera anywhere you want and no bullets are being fired.
When you press both of the buttons at the same time you shoot both of your guns at the same time and follow the camera like in any other game with dual wield.
Similar thing happens when you press only one of the buttons where instead of both guns firing, only one fires.
Now here comes the tricky part: if while you are pressing one of the buttons(1), you press the other(2) one as well, the already firing gun(1) locks in place and the camera switches to the direction the now firing gun(2) is facing and you can aim with that gun(2).
While this is taking place, you can only rotate to a limited degree(just point in one direction with your left hand and while keeping your hand in that direction see how much you can rotate and point with your other hand).
To switch between hands while in this dual mode, just release the button of the gun(1) you wanna aim with and press it again(1). That way the previous gun(2) will be the locked one and the newly pressed one(1) will be the unlocked one.
Really confusing, i know.
But it works without the need for an ability and with enough practice can be used to hit multiple moving targets at the same time.