Here is the eternal WordOfGod speaking to the eternal SpiritOfGod in agreement that He is willing to be manifested in the flesh to accomplish the will of the HolySpirit.
It is not two persons communicating about a plan.
This is the consciousness of God submitting to the heart of God to become the SonOfMan and accomplish the WillOfGod.
It is not any different than how each of us conceive in our hearts a desire and then consciously determine to pursue it.