Best isn't the right choice of words, the most offensive is dual wielding.
It is immensely effective mainly due to the triple spin power attack and the swipe with both weapons, which is why you should always have a dagger in your left hand. The speed of the power spin and the double swipe is dependent on the left hand weapon.
Wow, i never knew that, things are looking up.
Most of the damage comes from the perks and i found myself rarely ever using the off-hand sword for anything other than triple-power attacks.
In terms of fps, at minimum its a 35% increase, closer to 40 or 50% but definately not 100% increase unless youre spamming the triple-power attacks.
In terms of viablity its very hard to play it on master difficulty + enchaned difficulty mods if not impossible because monsters kill you in a few strikes and you have no way of staggering them outside of power attacks which are generally inferior to shiled bash for staggering purposes.