Well if you compare base on the old overpowered DKS enchantment formula, the damage difference might really be huge when comparing others styles to dual wield style, since level 10 enchantment could reach 100 damage extra, which mean 200 raw damage extra just from enchantment itself. I havent play FOV or doing any comparison in the DKS version yet, so I wont know how the end game weapons in FOV will be like.

But base on damage comparison I done before in the original ED, the conclusion is dual wield indeed provide a more stable damage output, where 2h damage output often fluctuate, due to the huge gap between its min and max damage. But the difference between 2 are not that much until double the killing time on the same mob. Comparison was done base on weapons in the same grade and similar level, as well as skill mastery level. This was tested with the earliest version of ED.

But with some later patches, there are some improvements on the 2h weapon, which provide 2h weapon aoe damage capability. Some tested and reported skill such as fatality, which by default a single target skill, will hit multiple targets if one is equipped with 2h sword. Same goes to normal swing from 2h sword as well.