“One heart” means that we are in a single desire to begin with.
This desire shattered and organized us into billions of people who are separate and distant from one another. In other words, in our current state, we have many hearts, or many desires.
Then, by correcting ourselves, i.e., by bringing ourselves closer together in a common intention to love, bestow and connect like the force of love, bestowal and connection that Kabbalists call “the Creator,” we come to perceive a different picture of reality: that there are not “many people,” but rather “one man.”
To the extent in which we bring ourselves closer to the Creator, adhering ourselves to the force of love, bestowal and connection, we come to perceive a single form of bestowal.
We then observe the state of unity “as one man with one heart,” in adhesion with the Creator.