To be perfectly honest, I find the maxed out rank 20 handgun ability to be a bit...underwhelming. Combined with the fact that we see Jackie dual wield, but can't do it ourselves, feels a bit disappointing. Thus I feel like it would be better to make dual wielding the Maxed Rank handgun ability. This would ultimately work one of two ways, but statistically they are the same thing.
Option 1: Linked equipment slots.
As the name suggests, this links the top two weapon slots in your inventory screen, allowing you to wield both weapons (assuming they are handguns) at the same time.
Option 2: Weapon Cloning
A bit immersion breaking, this ability simply spawns a copy of whatever handgun you equip in your offhand, allowing you to fire both. This copy only exists as an offhand equip, can't be sold or unequipped, and has all the same mods and stats as the main version. This is the more controlled and balanced option, as it technically isn't anything more than a fire-rate and ammo capacity buff.
Statistical effects of dual wielding.
Obviously this feature comes with some pretty big advantages; doubling the ammo count of your equipped weapon, potentially doubling the fire rate, and of course the cool factor. But there are penalties as well. Not being able to stabilize your gun with both hands means a reasonable drop in accuracy and fire rate can be expected (hence why dual wielding only "potentially" doubles the fire rate), and the biggest issue is reload time. Obviously needing to reload two guns would double that timeframe, but then factoring in the need to juggle the weapons to do so and that time goes even higher. This is the real penalty of dual wielding; if everything isn't dead by the time you empty both clips you may as well change to a different weapon, because nobody is going to give you that kind of time to reload. Likewise the accuracy dip pretty much necessitates using SMART guns like Skippy.
So overall, while the exact numbers can be tweaked with play testing, I think it would look something like this...
Functional Fire-Rate/DPS: +60%
Functional Clip Size: +100%
Functional Accuracy: -50%
Functional Reload Time: +150%
Option 1: Linked equipment slots.
As the name suggests, this links the top two weapon slots in your inventory screen, allowing you to wield both weapons (assuming they are handguns) at the same time.
Option 2: Weapon Cloning
A bit immersion breaking, this ability simply spawns a copy of whatever handgun you equip in your offhand, allowing you to fire both. This copy only exists as an offhand equip, can't be sold or unequipped, and has all the same mods and stats as the main version. This is the more controlled and balanced option, as it technically isn't anything more than a fire-rate and ammo capacity buff.
Statistical effects of dual wielding.
Obviously this feature comes with some pretty big advantages; doubling the ammo count of your equipped weapon, potentially doubling the fire rate, and of course the cool factor. But there are penalties as well. Not being able to stabilize your gun with both hands means a reasonable drop in accuracy and fire rate can be expected (hence why dual wielding only "potentially" doubles the fire rate), and the biggest issue is reload time. Obviously needing to reload two guns would double that timeframe, but then factoring in the need to juggle the weapons to do so and that time goes even higher. This is the real penalty of dual wielding; if everything isn't dead by the time you empty both clips you may as well change to a different weapon, because nobody is going to give you that kind of time to reload. Likewise the accuracy dip pretty much necessitates using SMART guns like Skippy.
So overall, while the exact numbers can be tweaked with play testing, I think it would look something like this...
Functional Fire-Rate/DPS: +60%
Functional Clip Size: +100%
Functional Accuracy: -50%
Functional Reload Time: +150%