The two fundamental metaphysical principles of essence and actus essendi are not instantiated in isolation in the real world. A subsisting extramental thing cannot not possess these two metaphysical principles at once and at all times.
However, in a subsisting extramental thing, the instatiation of the essence is not the same as the instatiation of the actus essendi.
In the line of essence, things that possess the same essence are said to possess the full essence and not some degree of that essence. There are no degrees of horseness, for example, because the essence of horseness is shared by horses by way of a univocal predicamental participation. One individual horse is as much of a horse when compared to another individual horse.
In the line of actus essendi, on the other hand, only God instantiates the fulness of the metaphysical principle of actus essendi. All other subsisting extramental things receive their actus essendi with the limitations imposed by the particular essence which defines what the thing is. The essence of God is the only essence which does not limit the actus essendi.
Participation in actus essendi is therefere said to be a non-univocal transcendental participation because the perfection which is being participated is instantiated only up to a certain degree in the individual thing.
In the words of Aquinas, “Nothing is assigned to a genus by reason of its act of being but by reason of its essence; and this is clear from the fact that the act of being of a thing is proper to that thing and distinct from the act being of anything else” (De Potentia, question 7, article 3, corpus).
The essence principle of humanity is the same in two different human beings, but the act of being principle of Plato is not the same as the act of being principle of Socrates for it is impossible that there not be, of one thing, one act of being. And, again, in Aquinas words, “each thing has within itself its own act of being, distinct from all other things” (Summa Contra Gentiles, book I, chapter 14, no. 2).
Thus, participation in essence is not the same as participation in actus essendi. In a subsisting extramental thing there always is a particular essence instantiated by way of a univocal predicamental participation and some degree of actus essendi instantiated by way of a non-univocal transcendental participation.