Dual wielding firearms isn't the smartest thing to do, no, but it actually does have some tactical viability. Granted we have but two eyes, or in some cases less, and can only aim properly at one target at a time, but it'd be entirely possible to use one weapon to suppress an opponent and the other for more accurate shots. Further, if you're not all that accurate in the first place, 2x spray and pray might actually increase your ability to take out an opponent.
In melee, dual wielding is about equivalent to god mode. As was stated, a parry followed instantly by an attack from the other weapon in almost unbeatable, unless the other guy has two weapons as well, heh. Takes a bit of coordination, but pretty much anyone can learn a move or two using two blades in a few minutes.
As far as game modding goes, I'd say it'd require a hefty hit to ranged accuracy, getting worse as ROF and weapon weight go up. Melee there should be an Agility and Melee Weapon requirement with no accuracy loss cause the guy's, you know, right THERE... (7 AGI and 50 skill would be a bit realistic.)
In combat it shouldn't be all THAT hard, knowing squat about the engine as I do. They have different grips for different 2-handed weapons, so I assume that's something you can alter. Given that it's just the model. There's also already that laser pistol that shoots 2 projectiles at once, from The Mechanic or whatever, so there's your pistols, and everything else would just need a faster firing rate.
You'd need to have custom weapons for everything you want to dual wield, of course. Probably the game can't accept another equip slot due to engine limitations, and it'd be a bitch trying to accommodate every possible combination of weapons. It'd probably be better to have some in-game mechanic (a-la the workbench) that would "combine" 2 identical weapons (averaging the repair condition) and spit out the result to inventory.
So yeah, should be technically possible, but good luck finding someone who'd actually take the job seeing as it'd take altering meshes, skins, poses, item creation, scripting, etc...