You do realise that that would require to nearly double the already enormous number of PC animation frames?
I believe someone worked on something like that once, attempting to reconfigure game mechanics so that you could equip two guns, just going the cheesy way about it without having to redraw the 923594 x 10^487 sprites Arcanum is composed of. Didn't succeed though.
You could simulate such a thing with scripting. Upon 'end combat' of the PC (presuming the game recognizes script attachment points on PCs), you could write in code that does a check for a flag (doubleweilding=true/false). If so, it'll simply play whatever animation the player is doing twice and return and RUN default. Maybe whip up a dialogue of some kind to flip that flag on or off....then, when attempting to duel wield, run a conditional check to see if the currently equipped weapon object type is is present in the player's inventory (barring that, a direct internal name check would be in order) and then if so flip the flag and run a loop script that checks to see when the weapon is removed from inventory (or dual wielding turned off). Then, of course, said script(s) would need to consume twice the ammo....assuming one is talking about firearms or other projectile based weapons.
Wouldn't it be possible to take a rifle, give it a custom inventory image (two guns for example) and adjust bullet consumption, firing speed and accuracy accordingly?
Yeah....actually....that would work. Just copy + paste the gun image and create a side-by-side image. Could supplement that with scripting so you could combine the two items in inventory somehow rather than purchasing a new weapon.