Per 
Item #: SCP-XXXX
Object Class: Euclid
Special Containment Procedures: Entities SCP-XXXX-A and SCP-XXXX-B have agreed to cooperate with the Foundation as long as we provide a new Host every ten years, and enough tests to keep SCP-XXXX-C morally conflicted. However, Guards are advised that the entities have sharp tongues that can extend up to 3 meters from their Host, and that SCP-XXXX-B enjoys causing pain and disfigurement.
Para-psychological assessments predict that, given the chance, SCP-XXXX-A and SCP-XXXX-B would cooperate to escape with little regard to SCP-XXXX-C's safety. The greatest risk will be in about 3 years, when SCP-XXXX-C dies, is consumed, and the entities reproduce into 4 new SCPs capable of crawling under doors. We don't know if they can accelerate this process, so the hermetic locks must be maintained at all times.
Description:
SCP-XXXX-A and SCP-XXXX-B belong a to parasitic species called the Xyp that specializes in Human hosts. While lacking a host, these parasites resemble cat-sized slugs ablo to compress and slide under doors. The two sexes (or rather, 'moralities') of these species always travel in pairs, searching for people that live alone, and attack while they sleep.
Once they ambush their prey, they latch onto the victim's neck, and one begins to bore into the human's shoulder while the other suppresses the screams of agony. Then it's the other one's turn.
When the process is over (after nine of hours of excruciating pain), the new host is left with two horribly disfigured shoulders, each with makeshift eyes and a mouth that can project tongues like bladed whips.
The entities can communicate telepathically with their host, and influence their behavior. Both cooperate to avoid detection from humanity by projecting feelings of alienation, and reminding the host that they are now hideous monsters behind their clothes.
In all other respects, however, the entities will oppose each other. Whenever the Host is conflicted about a choice, A pushes for the more moral option, while B pushes for the cruelest alternative. This clash of wills gave rise to their nickname, "The Angel and Devil in one's shoulders." Still, the host is the final arbiter, and their support of one of the parasites is what breaks the stalemate.
Each time an entity wins a moral conflict, it gets to spread a little further, gaining towards the victim's heart. After a decade or so, one of the two entities has gained a lot of ground, and the subject passes away due to a heart attack (having previously been manipulated into a secluded place.) Then the mouths open, and the parasites consume the body.
The bigger parasite (the overall "winner") will claim the head, while the other one (the "loser") consumes the rest of the body. With this added biomass, the entities are ready to reproduce via asexual division. The loser will divide into two copies of itself with little variation. The winner divides as well, but each of their new spawn takes one of the late brain's hemispheres into itself, assimilating personality traits and memories of their late host. We speculate this is why both entities seem to have developed such a humanlike psyche, and how the separation into "Moral" and "Cruel" developed in the first place, since a Moral brain will be assimilated into "the Angel", and a Cruel brain into the "Devil."
After their reproduction, the parasites split, with A-1 travelling with B-1, and A-2 with B-2, searching for two new hosts to begin the conflict anew.
Addendum: Interview with SCP-XXXX-A, "the Angel", while SCP-XXXX-B "the Devil" has been sedated. With only one of the parasites active, SCP-XXXX-C is at the mercy of its whims, and responds for the creature.
Q: You are the "Moral" one, correct?
A: Yes.
Q: How do you justify consuming intelligent life to survive?
A: Intelligent? Spongebrain here? Haha, I kid, I kid. But truly, it's conflicting, but it's like that Trolley's problem you are always testing about. One life weighted against two.
Q: You mean your Evil brother.
A: Well, Evil is relative. He must do as he is. And there is so much potential for Good to be done, if the Human we find has the potential for greatness. Why, just the one before… what a Saint I made of her! Ask around in her hospital, the number of lives she saved in that period…
Q: And the one before, the serial killer…
A: Had it in him all along. If only he had listened to me… but alas, he was nasty from birth, I'm sure of itttttt… gruad graad, He's Waking… You've been running your tongue, sonufobitch? What if I run mine??
[The interviewer, who had crossed the yellow line, retreats in haste and narrowly avoids a nasty cut in her face.]
