as far as i know equipping 2 weapons u will get avarage weapon damage from them, however you're benefiting from all the prefixes and suffixes in both weapons, if i remember correctly it boost basic attack speed. It means in current state of the game 1h+shield/catalyst is more profitable

The base damage on weapon doesn't add, but that's not important, because you get all prefixes, suffixes, and sockets of the two weapons, which is profitable in most cases because of stacking flat weapon damage, and % Material damage on this weapon

I think orionar was asking about how skill damage calculates when dual-wielding, not auto-attacks. Ori try swapping around which hands your weapons are in. If the skill damage values stay the same then they must use the average of both weapons. If they change then the skill must scale from only 1 of the weapons.

There is still a damage penalty when dual-wielding though. I equipped two pistols with nearly the same total damage (208-405 and 211-403), the damage in both character sheet and attack skill's descriptions reduced. I guess dual-pistols only advantage is in basic attacks (if you use it a lot).