>>532483138
>>532483250
>>532483335
I'm 95% sure you have to find both Glaives on a class that can equip them in order to do this as well.
One of my druids had both of them for ages but it was only when both of them eventually dropped for a demon hunter did I get "Warglaives of Azzinoth" achivement.
The achievement actually states
>WIELDER of a set of Warglaives of Azzinoth
a mage can't wield them