Holy Woo...in a previous conversation about this I'd suggested "Hardboiled" as the name for the dual-wield perk as well.

I'd say at least a 6 or 7 in Strength (firing a large pistol with one hand accurately isn't that easy), an 8 or 9 in Agility, a 75 in Guns, and at least several handgun-related perks including Hardboiled and a couple of pre-req's for it. Like I said, if they added it I think it should require a sizable investment by the character to get there.
To come at it from a "what would the perk require, and what would the act require" persepctive, how about:
Minimum 8 agility to even get the perk.
No strength requirement for dual-wielding itself per se,
but- since you're not using both hands on one gun, an automatic +2 to the STR requirement of whichever gun has the
higher STR requirement which then applies to both guns.
Reloading takes twice as long minimum- you've got to do it twice, plain and simple. None of this Hollywood "throw two magazines in the air, drop the ones out of the weapons, catch both flying ones simultaneously with the guns" silliness.
Accuracy, instant 40% reduction minimum (I'd prefer more like 60% min myself) and aimed shots aren't allowed- you wanna spray-n-pray, then start spraying and
start praying. Even in VATS, it's like melee in FO3: you aim for the target, with no option to target a specific part
of that target
I would at least like the option to do it.... Westerns are rife with gunslingers who carry revolvers on both hips (see my avatar). Since this game is going for cowboy influences, I'd like to have the choice.
Yes, westerns are loaded with dual-wielders. And people who fire 10+ shots between reloads, from 6-shot revolvers. And Bugs Bunny deliberate ricochet shots like pool players doing trick 3-rail shots.