I would say typically dual wielding outputs more damage than using a 2 handed weapon.
the average base damage of a two hander is 50 and a light weapon is 35 (galon grief vs santcified glad) your off hand weapon has a 25% penalty(also -10 acc) so your total base damage is 35+26=61. 61>50 obviously but there are some caveats
Most weapon buffs only apply to the main hand weapon(Charge weapon and its equivalents, agnimitra flame tongue, viv elemntal fury/blessing, ryn aurora arrow) These add anywhere from 50-100 damage and this is only applied to the main hand weapon which is important because the main hand weapon will have better pen/spell power.
The only exception to this rule afaik is the perk righteous fury which adds ~20 holy damage to both weapons
The eastern big blade does have 60 base damage and comes really close but it has 0 innate pen, -10 acc and -30 evasion. Before level 9 with its unique effect it would probably be the best bet for a two hander but dual wielding anything with a chaos blade is just fucked up.
There is also the fact that most powers that use your weapons(power wave, whirlwind, double attack) also make an off hand attack so two handers aren't even ahead in terms of powers.
Thats just how I've generally felt using dual wield vs two handers so I could be wrong.