There are several configurations of dual wielding:
1. matched dual wields. Both hands are holding the same type of weapon, I mean same length and weight. For example, two short swords or two long swords, or two maces.
2. mismatched dual wields. One hand is holding a long weapon and the other hand a short weapon.
3. Weapon+shield (you'll see why I counted this as dual wielding later)
Now, when you hold two weapons, theoretically you should be able to: attack with both hands; attack with one hand and block with the other; block with both. To implement this you'll need 4 buttons which is not realistic, so there needs some limitations and balancing. Following is my design of dual wielding:
With option 1, you are able to block with two weapons, giving you a better protection than blocking with one hand/weapon. However you are not allowed to block with one hand and attack with the other (just for balancing purpose). Also you are able to attack with both weapons at the same time, giving you double damage.
With option 2, you are able to block with one weapon (the short one, like in some fencing practice) and while you are blocking you can attack at the same time with the other, giving you high damage since the enemy exposed his vulnerability while attacking you. However, you cannot attack with both weapons since they are not the same length. The short weapon used for blocking will not give you a lot of protection either.
With option 3, you are able to block with the shield, but you can't attack while blocking (because the shield is in the way). However the shield gives you the highest protection, and you might be able to shield bash.
That being said, a good addition would be the requirement of timing while blocking with your weapon. This makes sense, because the shield gives you a large area of protection while you have to watch the direction of enemy attack when you block with a sword. The advantage of blocking with a shorter weapon in option 2 is that shorter the weapon, easier it is to time the block, because it takes a shorter time to place a lighter weapon in a certain position. Dual blocking in option 1, on the other hand, is slower and harder to time the block. When you are wielding two heavy long weapons, you might as well give up weapon blocking since it takes way too long.
How do you think?