Is the right-hand weapons DPS used? Is an average taken? Is it the min or max of the two?
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, ...!
Just open character stats and look up your Weapon damage. Its calculated from that. So your offhand also counts.
Two handed weapons often have better +% to certain damage types and more slots.
Dual wield offers more dps but damage overall can be lower in certain cirumstances so experiment.
But high end dual wands are better than high end two handed staff. Still its much easier to find good staff than to find TWO good wands :)