How can a hypostatic union between God and mortal be made incomplete?

[Erebus]

The Great Wheel is a planar cosmology which defines how the planes, worlds, and realms of the multiverse are connected to one another. Throughout this multiverse, there are various gods that exist which have their own cult of worshippers among mortals. However, they are prevented from interfering with humanity directly, as they are separated by dimensions. This forces gods to enter a realm through more creative methods if it seeks to influence humanity and the flow of history. Daeva is an umbrella term used to represent a collection of divine entities that have since entered the mortal realm and have become human. There are three specific ways in which a deity can accomplish this:

1. A god can construct an avatar, an artificially created human body built to the specific instructions and needs to contain it's soul. A god can then possess this body and use it to navigate the human world in order to use its power. It can also place pieces of its soul into numerous avatars, potentially dividing its power among thousands of bodies, depending on its power level and might.
   
   
2. A god can use its own cult as a way to enter the world through a method known as Theosis, a transformation process whose aim is likeness to or union with a god. Through this ritual, a worshipper who has proven its faith and devotion can offer up its body as a landing pad for that god's soul to enter, becoming one with its deity. The resulting individual becomes a perfect union between god and human, becoming intertwined in mind, body, and soul down to its very DNA.
   
   
3. A god is born into the mortal realm as a human through a virgin birth, done through a millennia long eugenics program to create a physical body with the perfect set of genes to contain the god's soul. This entity is considered simultaneously divine and mortal, possessing two complete and distinct natures at once. This is referred to as a Hypostatic union, in which a deity contains the essence of both a god and a human, having both natures but being one individual.
   

The third form is the most desirable, as it is the strongest and would allow God to exist in multiple realms simultaneously. In practice however, this has backfired. Instead of creating a perfect union between two beings, it has created a human being containing two souls, one God and one mortal. These souls are contained within this body but exist separate from each other, preventing the individual from reaching its full potential. How could this be the case that this perfect union be split apart?

 

[pmmg]

Moving to a plane where one cant exist.

 

[Fettju]

This is extremely interesting. Especially the second one.