The will of God is one. That is a direct inference from the doctrine of simplicity, which states that God is not made up of parts. Since God is simple, He is His attributes. One cannot remove anything from God without causing Him to not exist. God is like a binary — either He is, with everything that He is, or He is not.
In the being of God, to posit more than one will in God is to say that He is composed of more than one will. This introduces multiplicity into the being of God, and thus is an assault on the doctrine of simplicity. To say that God in His essence can undergo relations of authority and submission is to posit more than one will, and that is contrary to Scripture and contrary to Nicene Orthodoxy.
In the same vein, when we talk about the Trinity in His being, the divine nature is not a genus that is ascribed to the persons of the Father, Son and Holy Spirit. There is no such thing, technically speaking, of a "God-kind." God is qualitatively beyond anything in this world. The divine nature in its proper sense is God, the Triune God. There cannot be any taxonomy within the Godhead, which is a kind of ridiculous concept anyway!
All of these are biblical and part of true Christian theism. All these are non-negotiable truths which anyone who calls himself a Christian should hold to. All these form part of the legacy of the ancient church that even Rome (her non-liberal versions) did not reject when she rejected the other parts of Scripture. While it is technically part of councils to pronounce anathemas, it wouldn't be an invalid deduction to say that anyone who explicitly denies any part of these truths are heretics under the curse of God.
But if one were to be only fixated on ontology, then one will at best lose sight of the beauty of God's works, and at worst be an imbalanced "classical theist," of which there seems to be far too many nowadays. Alongside the immanent Trinity (ontology), we see also the concept of the economic Trinity (in outside-of-human-time "timed" and time-bound acts). Both describe the one Trinity, but the economic Trinity focuses on the works of the Triune God, from eternity past to eternity future. As I have said, God in His works is not timeless but everlasting. Therefore, while eternity is an attribute of God and thus of the immanent Trinity, we can rightly speak of an eternity(2) that is not timeless but speak concerning God's endless interactions in time.
When we open the pages of Scripture, what do we see and read? We see a God who acts in time, a God who seem to change (i.e. "repent") and so on. While we deplore the stupid interpretations of those who deny immutability, impassibility and so on, we must recognize that we have to deal with the narrative and descriptive texts and not superficially write them off everytime as just "anthropomorphisms" following which we platonize all of Scripture as a manual on "timeless eternal truths." If there is anything clearer in Scripture, it is that God acts IN history; God acts in time. God DID not wait for the incarnation to start working in time either. What else do we see in Scripture besides God interacting with people except that God interacts with God!
Psalms 2:7 tells of the decree whereby God the Father SPEAKS his covenant into being with God the Son. The text did not say that God decided that the Father and the Son would have such and such a working relationship. It says God SPEAKS the covenant into being.
I don't know about the "classical theists," but it seems to any normal person that speaking at least involves someone who speaks, and someone who is spoken to, in order for someone to speak to another. The one who speaks must will to speak, and the one who is spoken to must will to listen, and the two must be distinct wills. It cannot be one will, for then you have both speaking to each other and to themselves. In the context of the pactum, God the Father wills to speak to God the Son, and not the other way around, for God the Son does not will to speak to God the Father. It is the Father who says, "Today I have begotten you," not the Father and the Son saying "Today I have begotten you" to each other and to themselves, which is what would happen if both had one will between them.
Now, I can just hear the critics point that this text is a text about the eternal generation of the Son, and indeed it is. BUT... It is speaking not of the Father generating the Son, but of the Father speaking about the generation of the Son. The Father eternally generates the Son, THEREFORE He speaks thus to the Son. Psalms 2:7 is the revelation of eternal generation, not eternal generation! It is after all not that the Father speak and generate the Son into existence or some heretical notion, but rather the begottenness (Qal, Perfect Tense) happens in the past of this speech.
All of these is to say that from the Scriptures, we must acknowledge a very real dynamism in the workings of God among the three persons. In the economic Trinity, we can, indeed we must say that there are three distinct wills of some form of subsistence, otherwise Scripture makes completely no sense. To think that there is one will of God in the economic Trinity is to function like a modalist regardless of one's actual doctrinal profession. God the Father interacts with God the Son before the Incarnation, and it is ridiculous to think that the interaction does not actually happen because of an a priori idea of what "will" and simplicity must mean.
We must respect the three-ness of the three persons. God is BOTH one and three, not primarily one and secondarily three. While there is one will of God in the immanent Trinity, one should be able to say that each person of the Godhead has a distinct will. Otherwise, what kind of person is he if he cannot communicate except in unison? God is not three persons saying the same thing together, like 3 speakers attached to the same computer. But each one says differently. And as we have seen, it is in the pactum salutis that God the Son submits to God the Father, as the servant of the covenant.