Answer: Separation starts when additional desires unfold in Nukva and she realizes what she lacks to attain the level of the Creator.
I existed in Bina together with ZA, and He filled me with everything that was needed for my existence. But I started to desire to be independent, like He is, and to totally adhere to Bina. At this point, I discover and feel that I’m incapable of it. This state is called discovering limitations (Din). If I accept these restrictions and correct them, then I become like Bina.
This is precisely what we need to do: Malchut ascends to Bina, Bina descends to Malchut, and together, they work on correction. Malchut can raise all her requests to Bina, and Bina passes to Malchut all its strengths so that the latter receives all the necessary properties of bestowal.
Now it becomes clear why ZA has to have two Nukvas: Bina is also divided into two parts. Everything expands from Bina; it determines all that will take place in Malchut.
Bina is the force of bestowal that is revealed from Above. The Creator wishes to give everything, which is called the Light of Hochma (Wisdom). But He wants the creature to be perfect, to receive this Light, and to act as an independent character, as a guest across from the Host and not as a beast ruled by nature. This is the will of Bina. Hence, it has two parts: upper and lower, GAR and ZAT, where GAR is Bina itself and ZAT is its will to bestow to the others.
Accordingly, Malchut has to be comprised of the same two parts. She needs to have her own part, the corrected one, which doesn’t lack anything, is totally independent, doesn’t need the Host, is unwilling to receive anything from His table of treats, and is total perfection. Besides, she has to desire to bestow everything to the Host in response and to delight Him without any connection to herself.
Therefore, there are two parts in Nukva: one that establishes its independence and the other its equivalence to the Creator. These are the two Nukvas of ZA: Leah and Rachel in relation to the two parts of ZA, Israel and Jacob.
From the 3rd part of the Daily Kabbalah Lesson 10/4/10, Talmud Eser Sefirot