It is sometimes argued that the Christological formula of essence and person determines the way to understand person and essence in Trinitarian theology. The incarnate Son is a single person, the Person of the divine Son. But this Person exists in two natures. The church rejected monophysite Christologies; Jesus has two natures. And, the church rejected monothelite Christologies; Jesus has two wills.
On this paradigm, “will” must be an attribute of nature rather than Person. Otherwise, we’d be monothelites.
Applied to Triitarian theology, this means that God must have a single will, since He has only one nature. To say that the will of God is diverse is to detach will from nature, violating the Christological grammar, or, what is worse, to flirt with tritheism. “One will” thus becomes a test of orthodox Trinitarian theology.
Let’s grant for the sake of argument that the terms must be used in the same way in both contexts. Let’s even grant the will-essence connection. Even so, the two cases are not analogous. Jesus has a divine will and a human will because He has a divine and human nature. His divine nature (if we can, grotesquely, think of the divine nature stripped from the nature) is undifferentiated because He is the divine Son, not the Father or Spirit. The case is quite different when we talk about the Trinitarian essence, which exists only as Father, Son, and Spirit, and so is necessarily differentiated.
It’s perfectly logical to say: Jesus has a divine will because He has a divine nature, and a human will because He has a human nature. And, the divine will of the Trinity is a differentiated unity because the divine nature is a differentiated unity.
But I suspect the argument above is even less forceful than this. If we accept the essence-will connection as described above, then the divine nature born by the Son must be the single nature of the Trinity, and thus must be the one will of the Trinity. The two wills in the incarnate Son are a human will and the one divine will of the divine essence. But that makes fair nonsense of the way Jesus talks about His relation with the Father’s will: “My meat is to do the will of Him who sent me” must be translated as “My meat is to do the unified will that I share with the Father and Spirit.” That doesn’t qualify, but contradicts Jesus’ actual statement.
One might parry this by saying that Jesus speaks as “man” and not as God. That would yield an odd result: The “impersonal” human nature speaking an “I” and talking of obedience. Jesus is capable of speaking of Himself as obedient to the will of another, the will of the One who sent Him, the One whose mission He fulfills. They form a single purpose, but they form a single purpose because the Son’s food and drink is to do the purpose of His Father.