Correct me if I am wrong but you can't actually dual wield with great weapons, so no need to put them in the dual wield charts.
Another thing to take into account, and it makes things even I think, is the fact your probability to hit with two attacks is lower than with one. So it still makes the 2H weapon do more damage on average in most situations.
Flanking does screw things up as it's way too powerful and exploitable right now.
Dual wield is buffed again both relative to magic use and relative any other weapon style, by Larians ever-burning candles for weapon dipping. For dual wielded daggers, it doubles the base damage output for instance. You just have to be willing to go through the same ritual/exploit prior for every combat. An incredibly BAD game mechanic; repetitive, boring and unbalancing.
In general, all martial classes are HEAVILY buffed by the ludicrous ease of getting advantage (either by flanking or high ground) relative to magic-users (who in turn are both buffed by super permissive and unrealistic rest mechanics and nerfed again by barrelmancy/elemental effects). Classes such as Warlock and in particular Barbarians will be slammed hard if Larian's current homebrew is allowed to stay. Clerics too are nerfed in particular by the lack of a good way to leverage advantage on attack rolls (Shadowheart is built badly mechanically/thematically to further exacerbate this problem). The balance in D&D 5e is far from perfect, but Larian has fireballed any resemblance of balance in the game by seemingly defaulting to their DOS-mindset rather than giving D&D a honest go first. That they have failed to see this is a bit of a concern to me.
