My 2H Nightblade is making strides. Far better than him being DW. Overall 16.4 percent increase in dmg.
There has never been a record in history where someone is wielding 2 weapons in both hands. It's either a 1h and a shield (practical), or a large claymore (death-dealer, and probably suicidal).
But, if you want to create this fantasy, which MMOs have been doing for 20+ years now, you have to give us something. So, there are really two options from what I can see.
1) Give DWers better protection. Fictionally speaking, its a parrying tool.
2) Or it's a jabbing tool to find weakness. Increase the crit chance of the off-hand (could already be a thing, but not showing results).