First, accuracy to simulate the extra hit chance. Then dodge to parry.
Realism is not always a good guideline for improving gameplay, but in this particular case, both realism and balance are compromised. If dual wield gives a better chance to hit and a better chance not to be hit, it has no disadvantages whatsoever, so it's not that great from a balance perspective.
From a realism point of view, dual wielding is only effective at a very specific point in military technology history, when heavy armor and shields are uncommon for some reason, for example:
Armor/shields falling out of favor because of gunpowder weapons. (Renaissance Europe)
Heavy armor being a ticket to the bottom of the ocean (Golden age of piracy)
Metal being too rare for the development of heavy armor (Medieval Japan)
Without that, dual wielding an off-hand weapon is stupid. It does not come close to the defensive quality of a shield, it is too light to be effective against armor, or to be able to parry the kind of weapon used when heavy armor is common.
The rapier is my favorite weapon, and I have demo'd dual wielding at cons and meets, but in the universe of FE it just doesn't belong on the battlefield, unless we get units attacking from concealment.
And remember. When heavy armor became practical, ALL swords were used two handed (and not only by the grip) whenever the warrior did not have a shield.
