You'd think that a modder should be able to create a 'shield' that has attack instead of defense, a dagger graphic, and voila you're dual wielding (technically). If you just want to look cool with two weapons, and don't mind if your character functions exactly like a 2h wielder (doing more damage at the expense of defense), then congrads you're done. Well, getting the animations right is another problem entirely, that solution would just have your character stabbing with the main hand.. but anyway.
A more difficult question would be: gameplay wise, how can you differentiate/balance dual wielders vs. 1h+shield vs. 2h? Can the game recognize that a character has two (or more! think monsters with multiple arms) separate weapons and give him two separate weaker attacks (instead of a single attack with the damage of all weapons added to it)? Can you create a weapon that can be used in either main or off hand, letting you wield two of the same weapon? Can you restrict offhand weapons to only being used with certain mainhands? (i.e. two daggers is ok, bastard sword+dagger is not) Can you give dual wielders any other bonuses to differentiate them - perhaps some kind of 'parry' (defense) bonus - but that really just turns your offhand dagger into a glorified shield. Point being that getting the graphical effect of two weapons is one problem, getting it to actually function differently than a 2h and remain balanced is something else entirely (a problem that a certain immensely popular MMORPG has been struggling with for the past six years, so it's clearly not simple).