Well, Nakia had shorter hair and Okoye had longer hair, so even in that you could STILL differentiate between the two girls. When ALL the girls (or women because I'm not sure how old the Dora Milaje were supposed to be after Priest stopped writing them) in the Dora Milaje have NO HAIR AT ALL and look to have the same tribal markings on their faces, it was hard for me to tell them all apart from one another.
Plus it helped in Priest's run when T'Challa referred to both Dora Milaje by name...I don't recall though if T'Challa was ever shown calling any of the other Dora Milaje by name in Hudlin's or Maberry's runs other than the Aneka character.
Maybe other readers could see differences among all the Dora Milaje that weren't Nakia or Okoye, but I know I couldn't.