1. DAMAGE = X on HIT
2. DPS = Damage per second average
For ex this could be wrong but its for the Point of the explanation
Lets say Weapon 1 = 10 damage and Weapon 2 = 5 damage both with swing speeds of 1.0.
That means every second or DPS your Mainhand does 10DPS if it was the only weapon you used and your offhand would do 5 DPS if its your only weapon.
Now due to the nature of DW (i did not include the 15% speed ATKspd buff) you have to average your weapons = 7.5.
In this case your MH swings hits for 10 then offhand 1 second later hits for 5 and so on.
I could be wrong but at least this is a decent way to explain how it works. Let me know if that helped
Edit - for Diablo 3 sakes you want to ideally always have two weapons of Same Attack speed and Ideally damage range I think. Actually this is the part I am slightly confused on I know how damage works but the attack speed thing still confuses me a tad.
Special Ability 1 uses 100percent of your weapons damage really means
weapon 1 = 10
weapon 2 = 5
ability hits for 7.5
what you want instead is
weapon 1 = 10
weapon 2 = 10
ability hits for 10
or as close as possible this is why many find it hard to believe that using a far superior MH weapon + Shield can sometimes out damage DW including a not so great offhand weapon.
Edited by nycste - 5/21/12 at 11:21pm