The novel’s “Twins” motif was dropped in the film, but was prominent and even crucial to the book’s narrative. The novel presents a spiritual relationship between mass murderer James Vennamun and his beloved, but mentally-emotionally damaged, brother. The damaged brother dies, James becomes “the Gemini Killer,” then himself dies… then, through demonic intervention, he enters Karras’ resuscitated corpse. However, in an act of transcendent love, his dead brother searches for James and eventually finds him in Karras’ body. When contact is made, the “weaker” but saintly brother convinces James to end the killing spree, freeing him from the demon’s grip and from Karras’ body. Finally the “Twins” are once more restored to their prior unity in love – as they had been in their mortal lives.
The Twins theme is duplicated in Blatty’s cosmological myth of “the Angel.” We human beings turn out to be the Angel. Our origin was as two primoridal light-beings living with each other in loving unity. They are “One though Two”. They are both spirit-Twins, united in primal love. Then one of these radiant beings decides to live its own life, including pain and evil, as Creation. The Other tells its “Brother” about the burdens this will entail, yet we – the Divine Brother who desires Becoming – insist that we must seek our destiny in “Manifestation.” The Other agrees to this “Great Split,” saying that in the future both sphere and light-point will be once more united in love: “Hasten the day.” In becoming the manifest, created world, the primal divine light-point – we, human beings – have become the Angel seeking its divine origins who appears in Kinderman’s dreams.
We also function as the Creation-Upward-Groping to its godly origins, an idea which forms the essence of Father Lankester Merrin’s mystical vision in the original novel. The parallels to the Gemini/Vennamun case are obvious. The earthly twins’ life story duplicates the cosmic story of Sphere and Light-Point – both begin in love. One (James Vennamun) seeks its destiny in the material world.And of course, the term, “the Gemini Killer,” as James Vennamun is known, itself connotes twins. The other finally makes its own journey in order that both can once again be bound together in the unity they had before their “Great Split.”
Another “Twins” parallel is between Dr. Amfortas and his Double. At first taken as a hallucination, it soon becomes evident to Amfortas that his Double is his spiritual Twin – in this case, his “good” Angel in opposition to his egoic personality which is deeply implicated in the Gemini’s crime spree. While there is no explicit reunion of Amfortas with his Double-Twin, Blatty does supply implicit hints that all is not lost. The reader is left with the impression that, with a certain amount of penitential cleansing (perhaps in a Purgatorial or “Bardo” state), Amfortas and his Good Twin may at last be reunited as some kind of “blessed spirit” in the next life.
Finally, there is the “Twins” configuration in the Kinderman/Father Dyer dyad. These two characters seem to be Blatty’s expression of an idealized “Twins” relationship, in contradistinction to the flawed dyads exemplified by the Gemini twins, and Amfortas and his Good Double. The Kinderman/Dyer “twinship” is is a shadowy, earthly example of the heavenly union-in-love brotherhood found in the “Pre-Great-Split” unity of Sphere and Light-Point prior to the world’s creation. There are probably several other “Twins” motifs present in Blatty’s novel. The examples in this articles are just the ones I’ve detected thus far – or, at least, imagined.