This turns on at least two questions:1. How much do you care about reality? Its actually pretty hard to effectively use two full sized weapons in off hands; most of the time its either two smaller weapons or a smaller one and a normal sized one. And as someone mentioned near the start of this, one of those two is usually used as a defensive weapon (classically, the smaller, but I actually found in my brief time doing it in martial arts training a long time ago that I often got better results out of blocking with the longer blade and following through with the short.) Its possible to fight with a brace of swords (i.e. two arming swords or equivalent). Its even possible to occasionally do a strike/follow-up combo with the two. Its hard enough to do that its been very, very rare. So while double damage is a possible consequence of two weapon fighting, its an unlikely one on multiple grounds.
2. How much do you care about game balance? I know of multiple games that permit two weapon fighting in one fashion or another, but usually either the price to do so is high (in RuneQuest you were throwing away your primary defensive capability if you struck with both weapons, one would be delayed after the other, and there was no assurance that just because one hit the other would) or the benefit is mild (13th Age gives you a reroll if you roll a 2 on the D20. That's it).
So there you are.
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