Melee attacks using 2 weapons become 20% faster (both the animations themselves and how quickly the power bar fills).
The highest of attack and defense modifiers are used when wielding 2 weapons. IE: If the offhand weapon has a 0% attack bonus, and the primary has 50%, 50% will be calculated for both weapons.
Characters with a weapon equipped in their shield slot will enter the dual wield state with a different stance and animations.
The power bar fills between each attack, alternating between the main and offhand weapons.
To hit check for off hand attacks use the lower of Dual Wield skill and the attack skill of required by the weapon.
Your character will benefit from spells on both weapons provided they do not overlap.
The effectiveness of the off-hand Rend/Vuln level is determined by your Dual Wield skill. For ex. if you are max heavy weapons and your DW skill is 220. your off hand heavy rended weapon will only rend the equivalent of Vuln II.