The Darkness is consciousness. Remember that One's power with the Darkness is only limited by their own consciousness. What if someone could multiply their own consciousness? When Witness speaks it is always in multiple voices. He always uses "we" when referring to himself. Possessing multiple minds in one brain would be impossible, but the Witness doesn't have a normal head. I believe the Witness has used the Darkness transfigurate his own head into possessing multiple brains and therefore multiple consciousnesses. With this his control over the Darkness would be unmatch to all except for the Winnower. Just imagine if he could further expand this ability until he his nothing but a mass of heads. Now that sounds like a unique raid boss. Be careful Guardians. We are fighting against one hell of a narcissist.