The decretal will of God is volitional (the faculty of willing). Hence, there is but one will of God. While we may speak of will of disposition, will of precept, etc., these are but nice ways to understand the ineffable.
The usual way "two wills" are distinguished without introducing contrariety into the one will of God is to acknowledge the simple point that the word "will" is used in two different ways.
In one sense He is said to will something volitionally. This is God's will properly speaking. "God works all things after the counsel of His own will." Eph. 1:11
There is also the extended use of the word "will" when a certain course of action is said to be the will of God. "This is the will of God concerning you, even your sanctification." In this latter sense the word "will" is being used morally, not volitionally. 1 Thess. 4:3
The two words may be used without contradiction or confusion if we keep in mind that the secret will refers to what shall be and the revealed will concerns what should be.
The precepts of God are just that: what man ought to do. Prescriptions. Thus, we use the phrase prescriptive will. We speak of God's prescriptive will understanding that God's precepts are not the faculty of God willing, for what God wills (volitional) cannot not happen. These precepts are but what God commands that all should do.
Again, the word "will" strictly means something which is volitionally determined. Reformed theology understands that God's will always comes to pass and is never frustrated so far as the futurition of events is concerned. Psalm 115:3 and Ephesians 1:5,11 should suffice to show that the proper sense of "God's will", as in that by which God determines and effects what shall and shall not be and come to pass, is invariably accomplished in its fullest extent and in its minutest detail.
On the other hand, there is also a use of the word "will" which does not accord with its strict meaning of volitional determination, as when an action is said to be God's will, that is, God requires that a certain action should be done by men. This does not necessarily come to pass because God may not have willed it to come to pass. Hence it is a less than literal or improper use of the word "will." Note, "improper" does not imply that the word should not be used; it only refers to the fact that a word is not being used in accord with its strict meaning.
God’s will is, and rightly ought to be, the cause of all things that are. For if it has any cause, something must precede it, to which it is, as it were, bound; this is unlawful to imagine. For God’s will is so much the highest rule of righteousness that whatever he wills, by the very fact that he wills it, must be considered righteous. When, therefore, one asks why God has so done, we must reply: because he has willed it. But if you proceed further to ask why he so willed, you are seeking something greater and higher than God’s will, which cannot be found.
What Reformed theology denies is that a man’s will is ever free from God’s decree, his own intellection, limitations, parental training, habits, and (in this life) the power of sin. In sum, there is no such thing as the liberty of indifference (the so call free-will of Arminianism, open theism, Romanism); that is, no one’s will is an island unto itself, undetermined or unaffected by anything. According to Reformed theology, if an act is done voluntarily, that is, if it is done spontaneously with no violence being done to the man’s will, then that act is a free act (liberty of spontaneity).
Calvinists are not “free willists.” They assert indeed that man is free—that he is a moral agent not caught up in the wheel of things or determined by mere natural antecedents. But they apprehend that this is something else than freedom of the will. Man is free, that is, he can under ordinary circumstances do what he wills to do. But the will is not free, i.e., there is no extra-volitional vantage point from which the will can determine itself. Man’s will is a property of his nature, which is what it is by sin or by the sovereign grace of God. All of which leaves responsibility fully grounded, for nothing more is required for holding a man accountable than his acting with the consent of his will, however much this may be determined.
Thus because God decreed (decretal will) that all things would come to pass according to the nature of second causes, which means that in the case of men they would act freely and spontaneously, whatever sin they commit proceeds from them and not from God. God does not sin, nor is He the author of sin. Only self-conscious, self-determining, rational second causes sin. God’s freedom is not a threat to human freedom, but the very presupposition of the latter’s existence.
AMR