Lauge Nielsen on 7. Jun. 2017.
I'm pretty sure you will only get the Warglaives achievement, if you obtain both glaives on a character that can actually equip them, ie. Warrior, Rogue, Death Knight, Monk and Demon Hunter. Farming them on a Paladin or a Hunter or any other class won't give the achievement and thus won't allow your Demon Hunter to transmog the glaives after the Timewalking Black Temple event.