Well i have no experience with 3D in fonline whatsoever (so no idea if one can actually have 2 weapons in one animation), but this is how i would go about the implementation of the dual wield.
Let's assume it works only if you have same 2 pistols in both hands:
First you need to add some button into GUI or hotkey to switch between normal and dual-wield.
Then you need dual-wield version of the pistol - that means item with dualwield attack animations, same stats just with AP costs for dualwield attacks/reload.
So now that player has one 10mm pistol in each hand, when player presses the dualwield shortcut, the pistol in primary slot gets changed via script into the dualwield version (perhaps some animation plays between those two states)... the same thing should also happen the otherway around i guess.
Now when player attacks you just have to check ammo in the pistol in secondary slot (also substract it) and add second hit calculations (hit/miss, damage etc. for the second pistol) via script magic.
That's about it.
Next issue would be adding shitloads of checks for every situation when the dualwield version of the pistol item might leave the primary hand slot (death, critical drops, move to inv or another slot etc.) so that it gets reverted to its normal version again. Same goes for second pistol leaving its hand slot, you would have to revert the pistol in primary hand in that case too.
