For a complete explanation, please look at my answer to this post.
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, ...!
