There's nothing that shows you the total bonus damage you got for now, maybe at release. Every damage bonus are additive, so a +15% melee damage has the same value than +15% attack damage, the only difference is what it affects. The only damage bonus that are multiplier are bonus that adds damage as a specific damage type (they're all related to the mage/ailment archetype).
The average damage showed in the character tab is the one from your basic attack combo, which is a skill, and like other skills the average damage doesn't take into account the base attack speed of the combo or the number of hits. So if you dual wield pistols for exemple, the base skill will deal something like 60% of your damage and it'll looks like you just lose damage while in fact your real dps will go up, due to the high attack speed of this combo.
Your skill damage are based on the average between your 2 weapons when you dual wield, so exept to have a different basic attack combo or stacks affixes/sockets there's no real advantage to dual wield.