I don't think there is any foreshadowing involved. The Horror/Tragedy/Drama is already long running, ever since the beginning of the story.
The whole point of this story is the warped perspective of the human called existence. Truth is, in logical terms, the definition of existence.
However, what we as humans perceive as "true" and "real" are just shadows cast by their true existence.
This is because of relativity, which says that energy equals mass times the speed of light squared.
This means matter is just energy, and the human senses are unable to detect most of the energy around us,
meaning we're also basically blind to anything that does not manifest as matter, yet still exists as energy, which is the same.
This led us to be wary of "supernatural" phenomenons (which are of course natural, but we are unable to detect them),
making distinctions based on outcome (e.g. ghosts, gods, demons, miracles, spirits, ether, natural energy,etc.) as we are even unable to grasp the shadow fully that is cast upon us.
Since this story plays with the whole inhuman sphere (ghost/god/demon/spirit/whatever) and how it is fitting together (i.e. in the reality of existence, warped by the human perception) as well as the theme of possessiveness,
there are three ways that can play into the existential theme:
1. The denial of your own (separate) existence
This is a common theme across literature and psychological problems, with one party denying their own existence separately to someone elses, meaning that they cannot exist apart. This can be due to reliance on the other party or possessiveness.
In extreme cases, the person denying itself can either fuse with the person in question or sacrifice its life for the other person (the "live on in your heart" trope)
2. The denial of the others (separate) existence
This is the exact opposite to the previous case, usually depicted by someone being extremely possessive of someone else,
sometimes to the point that they follow their every step (stalker) and/or hindering their social interactions with others (if done violently, we get the usual Yandere/Yangire character)
In extreme cases, the person denying the others existence can force them to (partially) fuse with them (in spirit/and or body) or to turn them into a possession of theirs by killing them (and sometimes consuming them)
3. The denial of every other existence
This is the famous "being in their own world"-trope which ignores to a certain grade the existences of others interfering with them.
In extreme cases, this can lead to either seclusion from them world they live in into a void or to the complete annihilation and/or rebuilding of the world you live in (e.g. NGE: End of Evangelion, where Shinji denies the existence of every other human except for Asuka, which leads to the death of all of humanity and a new start of the world with only the two of them).
Funnily enough, Mirai Nikki does a brilliant job of exploring all 3 forms of existantial crisis'/denials, with Yuno first denying her own existence, murdering herself and wanting to make Yukiteru the new god, then denying Yukiterus existence when she fails her initial plan and trying to start over by killing him and becoming god again, and in the end the denial of every other existence except of Yukiteru and Yuno when Yukiteru becomes the new god and holes up for thousands of years in a separate world of void, in which he stares at his phone displaying the last message from Yuno prior to her death.
Since we have already grasped quite a bit of how they interact with others and how they view themselves,
it's highly likely that we will see one of two scenarios:
1. the two gods/demons slowly consume the spirit/life force of their respective others/targets to fuse them with their existence, leading to the decision of the two humans between denying their existence to save themselves or offering themselves up to become part of them
This would be the bad end.
2. the denial of every other existence, meaning the "eternal" seclusion of Yuu and Chiyo from the world that denies them coexistence
This would be the good end.
In both cases, the acceptance of an existence outside of the realm of human perception is coupled with the loss of something, and bound to a decision of what is more important or better said, more true to the main character, because we are not talking about pure feelings but to accept a whole new reality and life within it.
Depending on the possible twists and implications, this could be even more interesting considering the fight of the human mind against insanity during adaptation to the new circumstances, which shouldn't be easy for the main character.