@Ayeba Sorry, I'm not interested in crafting rules to suit guns akimbo. More interested in memorable characters that ooze personality.
However...
I'll say since Multiattack with pistols is already possible if you have something to get around loading, I'm open to just fluffing that Multiattack 'looks like' dual pistols under certain conditions.
Artificer's Repeating Shot (which pulls ammo from thin air) infusion + Gunner. This would be the best way. It would require 2 attunement slots though. Repeating Shot gets around the free hand requirement of the Ammunition property. Gunner gets around loading and enables Multiattack.
Gunner + Thief's Fast Hands. Without a way to get around the free hand requirement of Ammunition, you would be doing more than one free Use an Object as you alternate stowing and loading one at a time. This is similar to the old solution for dual throwing builds, before the feats and maneuvers were released to accommodate that style. It essentially eats your Bonus.
Gunner + DM Custom Gun. I might drop revolvers and such in the future but I wouldn't base a character around that possibility.
I'd have to think about a refluffed Sun Soul or Crossbow Expert, but its probably fine as long as the damage and ranges were adjusted to match those weapons.
edit: Actually for that first combo with artificer, the Gunner feat would be a bit redundant.