The current weakness of a 2H DD compared to a dual wielder is simple: The DWers get to have 2 weapons, some of which have amazing stats (using taming sari's as examples), whereas 2H get to use a 2H weapon and a pathetic grip. Recently, JSE weapons have been able to be augmented, and the 2H weapons have been endowed with impressive amounts of accuracy. Relics also have a lot of accuracy on them, but their obtainability is anything but practical, so let's exclude these from the conversation for the time being.
So before the most recent update, the most accuracy you could probably expect on any 2H weapon was around 30, combined from any DEX boosts and accuracy boosts. This was about the same for Dual Wielders. The difference is, they get to use another weapon that has similar (maybe even more) accuracy from their offhand. Comparatively, 2H's best grip in terms of accuracy is +10 to my knowledge, with Tzacab grip as my example. Now, let's assume I'm wrong and assume there's maybe one with +15. Is that a game changer? Absolutely not.
If you combined Duplus Grip, Bloodrain strap, and Tzacab grip, you'd have ACC+16, ATK+10, DA+3, STP+6, and HP+50. The combined stats from three of the better melee grips still do not match a taming sari with poor augments in any department except Store TP and HP (which is not as important to begin with).
The bad stats on grips was explanatory enough back when weapons didn't have extraordinary stats on them. But now they do, and the stats on the grips have not been brought up to par. Introducing a few "legendary" grips could help bridge that gap between 2H and dual wielders. Example: ACC/ATK+15, STR/DEX+10, STP+4 or DA+4 or even TA+2). The most logical solution for this would be to introduce one in Sinister Reign, or add a drop to one of the T2's in Ru'Aun that can be augmented like the weapons. Will it fix all the problems? No. But it is a step in the right direction.