All two-handers got +1 PEN with patch 1.2 to improve them for balancing reasons.
Because of that this special two handed battle-axe should also get +1PEN compared to a one-handed battle axe. If it hasn't that's clearly an oversight and should be reported and fixed.
Of course it's not useless, far from it, but this is only due to its unique enchantments, not because it's a two handed axe.
THis case is a precedent for all the folling ones where weapons of a certain proficiency but different handling style get introduced (think of Abydon's Hammer). For example: if Obsidian introduces a unique great sword that could be wielded with one hand once you reach 20 MIG, it should get -1 PEN compared to "normal" great sword once you use it with one hand only.
If Obsidian introduces a two handed sabre that should get +1 PEN - because it occupies both hands.
Edited by Boeroer, 13 August 2018 - 12:53 PM.