The part
written here is for flavor and balance reasons. Mostly flavor.
You have guns in both hands, and then you just fire the same gun in the same hand twice!? Why wield two then? If apparently you are dexterous enough to fire at rapid speeds anyway and ignore your other gun lol.
But I agree it might be unnecessary.
Adding your suggestion
seems cool but it is too powerful. Essentially our good 'ol pal the gunslinger gets the Multi-target Attack I and Multi-Attacker I feat all in one feat via the option to choose one or the other. With the mere cost of 3 feat points.
If I’m understanding what you wrote.
Maybe instead the disadvantage incurred when declaring to attack twice can be negated via the Multi-targeting feat?
I’ll have to think about it, cuz I’m a fan.