even though some of the animations are both hands at once, your damage works as if you shot one at a time
things like +damage, elemental, and increased speed are only for that weapon; the weapons tooltip calculates those to give each weapon their specific DPS.
the main benefit for dual wield:
mf% and Dexterity, which are for your character, not your weapon.
a flat 15%** speed increase (which can be helpful for hatred generation)
**so 1.00 -> 1.15, or 2.00 aps -> 2.3
the draw back is you should never equip a weapon that is < 74% the DPS of your stronger, the 4.0 is far too weak, so as you shot right hand, then left hand, the weakness of the 4.0 is costing you avg damage. Your char DPS listing is trying to tell you that, (intended)
also with patch 13, quivers are much better for damage, make/find them with as much +dmg /dex as possible. i.e. "single wield"
two craftable level 13 1h xbows or just one + journeyman quiver.
as for what I recommend, see fig 1.
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