For a while, just put aside the terms like "substance" and "essence" but take an analogy: rays of the sun say to plants: you should love and worship only the one sun, because unless the sun, you will all wither and cease to live!" But, can the sun provide living to the plants unless it sends its rays? No. Therefore, the rays should be necessarily co-loved and co-worshiped with the sun (by the plants, I mean).
The same applies in theology but to an incomparably and other dimensionally greater degree of unity and inseparability than the physical analogy of the sun and its rays, for we deal in theology, in speaking of God, with the sphere totally devoid of corporeality, to the effect that it is far more impossible, even other-dimensionally impossible to imagine a division of unity of the Father and the Son that it is possible with the physical sun and its rays.
Below I shall demonstrate that God's new covenant with humans entails that humans cannot engage in this new covenant, cannot fulfil its precepts unless through Jesus Christ, who is necessary for uniting God in terms of this new covenant. But I shall also demonstrate that neither God can engage humans in His new covenant with them unless through His Logos, the Latter also called Jesus Christ after the Incarnation. Now, if God is impotent to engage humans in His new covenant with them, that is to say, if God is impotent to save humans but through His Logos, then given the absurdity of the statement "God is impotent to save humans" and the truth of the statement "God is potent to save humans only through Jesus Christ", makes Jesus Christ also God by a [theo]-logical necessity.
God is one, and this one God makes a new covenant with the humans through Jesus, so that only by following the statutes of the new covenant, the new commandments of Jesus (who does not say, by the way, that those are God's commandments and that He only transmits them, but calls them "My commandments" and reveals Himself as not just a trustee of them, but the very Fountainhead of them (cf. John 13:34)), can humans say that they are pleasing God. Now, those commandments, as Jesus says, is impossible for humans to fulfil without Jesus Himself helping them, which He expresses even more intimately and inseparably, comparing Himself to wine and the disciples to twigs (John 15:5).
Thus, nobody can come to the God, i.e. fulfil the novel impossible commandments that make us compatible to God's new covenant with us, unless through Jesus.
But, now see, that it works double ways: not only we cannot please God unless through Jesus, but neither God can help us to please Him unless through Jesus. Thus, Jesus is necessary for the new covenant of God with humans to function both for humans and God. Just like God not only does not but cannot create universe unless through His Logos, so also God cannot help us pleasing Him unless through His Logos, who after the Incarnation is called also Jesus. If God cannot create the universe without His Logos, then Logos is incomparably greater than the created universe. But can Logos still be created before the creation of the universe, so that we may hold a theory of a double creation (as weird as this may sound): that God first creates His Logos, and then He is necessitated to use this Logos in creation of the universe. But this is an absurd hypothesis, for if God cannot create even the universe without His Logos, then how could God create Logos who is incomparably greater than the created universe being its Creator alongside with God?
Because those suppositions are absurd and mythological, the true theology holds equal divinity of the Father and the Son/Logos and gives Both the same worship (John 5:23) through their co-eternal Spirit who teaches humans this equal divinity and Lordship of the Father and the Son (1 Cor. 12:3).
Now, given all that, you can wisely and safely apply also philosophical categories and terms of "essence" and "substance". Like the sun and its rays comprise one reality or substance, insofar as they cannot be imagined separately but imply each other, so Father and the Son are the same reality, in that neither can be imagined without the other, for They imply each other. Yet, while identical in reality, or "essence" you may say, which is another Latin synonymous term, They are also different for Father is the eternal Source of the Son and not vice versa. Thus, the true theology holds both absolute and unsplittable identity of Godhead together with absolute and unmix-able difference in the same Godhead.