Righteous Fervor from Oathkeeper is not DW, Shadow Strike, Ring of Steel works with 2H too. Nightblade having Dual Wield skill line doesn’t mean you’re forced to DW. Acid 2h Amarasta’s Blade Burst is doable but in ranged 2h version is way more accessible.
You can play 2h Acid Shaman if you want, might not be super optimal. You can craft this at high level for conversion and resistance reduction Unique prefix - Item prefixes - Grim Dawn Item Database Maybe this for leveling if you’re Occultist + Shaman Putrid Necklace - Items - Grim Dawn Item Database
Also there various 2h melee Acid builds but usually more gear dependent, DW are more accessible. For example my Acid Reaping Arc [1.1.8.1] Winter King Might be an Acid Witch Hunter | CR 5:39 (CR 4:39 / SR90 when modified and piloted by grey-maybe) - #55 by tqFan Or more standard Shadow Strike.
There are also a lot of 2h Acid casters like 2h Acid Winter King’s Might, Phantasmal Blades, Dreeg’s Evil Eye…
My 2h Acid Primal Strike Conjurer although the weapon used is somewhat fantasy GrimSheet: Endgame Build Table - ETE edition [v1.41] - #363 by tqFan