For a complete explanation, please look at my answer to [this post](http://gaming.stackexchange.com/questions/85258/how-does-the-skill-damage-work/85799#85799).  

When you only have **1 weapon** equipped (no offhand) then the damage will be completely determined by the dps of that weapon.    

When you have **1 weapon and 1 shield** equipped, the damage will be the same as if you didn't have the shield equiped (so the same as above).    

When you have **2 weapons** equipped, the damage will be completely determined by the *dps of your right-hand weapon*. However, since you have 2 weapons equipped it will give you more damage than if you have just 1 weapon equipped, say, as a  sort of bonus. I believe the amount of 'bonus' to be equal to `floor(dps of righthand weapon / 10)`, however I couldn't do many tests on that statement so it's best to take it with a pinch of salt. 

So: always equip your highest dps weapon in your right hand and depending if you want more dps or more survivability you can choose a weapon (any weapon, stats don't matter *) or shield as offhand.   


*except if that weapon gives you bonuses such as dual-wield modifiers, %damage, ...!