I fucking told ya, seriously, don’t click on this unless you want to RUIN the game, not just spoil it.
Here we go.
The One and The Other One are two immortal beings that have existed as long as humanity have and probably before. They are opposites, but they are intertwined. The One is drawn to Creation, The Other One is drawn to Destruction. The One has confidence in humanity and is sometimes able to convince The Other One to go along with her schemes to create stories to attempt to better humanity. I believe this is because The Other One is truly in love with The One, and that trumps his distaste and pessimism for the potential of humans.
One of the stories They created is the Jesus narrative. They thought this would be it, the one to finally put humanity on the path of bettering itself. Obviously it didn’t. This causes some disillusion for The One and she spends the next two millenia or so alternately wandering the earth and being “dead”. Apparently, the harsher the death, the longer They stay dead but They always come back, BUT The One says They don’t come back from being cremated. (How do they know this, I don’t know, but I THINK i remember there is one clip where The One hints that there once were more of Them than just these two, but now these two are all that remain.) This is important for later.
Fast forward to France during WWI, The One comes about Marissa dying in a barn and takes her form. She is reinvigorated by her beauty and youth, and spends the next few decades modeling and traveling in Europe, eventually reveling in this brand new art form, the Cinema. She can tell that the Cinema is something special, a completely new art form unlike any other, and being a lover of creation, she wants to take part of it. She tracks down Arthur Fischer, considered to be the best director at the time, and auditions to be in his movie. Unfortunately, Fischer’s career is on the decline and he’s no longer the artistic genius she thought he was, but no matter, she’s in the biz and she loves it. This first movie is AMBROSIO. While filming AMBROSIO, The Other One tracks The One down and assumes the form of the actor playing Satan. He’s mostly there just because he loves her and wants to be near her, but She’s pissed because She knows that He doesn’t approve of her philosophy of creation. However, The Other One lets her have this movie and it finishes without any problems.
The One (as Marissa) gets involved with the Director of Photography on AMBROSIO, John Durick, and follows him to New York to make Durick’s directorial debut. This movie is MINSKY. Filming starts of well, Marissa hits it off (way off) with the leading man of the movie, Carl Greenwood, and Durick digs it because he’s a hip artist and free love and whatever (it’s hinted he’s kind of a perv). However, The Other One tracks The One down in New York and His patience is starting to wear thin. He sees Durick as a hack, that this movie means nothing, and wants to get The One to give up on this new obsession with the movies up and just be with Him. She refuses. The Other One then kills Carl Greenwood and takes his form. This really pisses Her off and she “accidentally” kills fake-Carl via on-set accident with a loaded gun. Carl’s body is then cremated and The One thinks that means she is free from The Other One forever. This ends the production of MINSKY.
At some point during the production of MINSKY, The One reveals her true nature to John Durick. Durick is horrified and The One winds up killing him because of his reaction. Durick then disappears for the rest of the production and The One (as Marissa) tries to direct the rest of the movie herself. (There are a few scenes where the crew is like “should we go without John” and Marissa just says, “let’s just do it” and directs the scene herself.) It, obviously, goes to 
with the aforementioned interference of The Other One.
The One then decides to retire Marissa and lives as John Durick for the next couple of decades, trying to make movies but each one turns out worse than the one before. Her enthusiasm and belief is running out, and being alive for so long means she is “running out of time”.
At some point, the actress Amy Archer watches the footage of Carl Greenwood dying. Through this footage, The Other One is able to live again as Amy. It turns out this new medium of Film is another way for The One and The Other One to transfer themselves into new bodies. They no longer have to destroy who they want to become. They can Create and live on through the movies.
This allows The Other One to see The One’s viewpoint of the importance of creation (and the power of film). The Other One (as Amy) tracks down The One and sees that The One is at the end of her lifespan. The Other One offers his help to The One to make one last movie, the movie that will explain everything, and will also give her the vehicle for true immortality. This movie is TWO OF EVERYTHING.
During the production of 2OE, The One is both John Durick and Marissa Marcel (this part confuses me, because this ability was never hinted at before but as far as I can tell, this is what is happening). The Other One is Amy Archer, who plays the role of Isabella Hessenberg in 2OE. The wear and tear of being two people while also having “lived” for so long without “dying” and “resting” is causing a huge toll on The One. She starts to literally fall apart, to the great concern of The Other One. Eventually, she is unable to finish the movie and agrees to die on film. The Other One burns her alive. Upon watching the footage of Marissa Marcel’s death, we (the viewer) become The One and The One becomes us. She has achieved true immortality.
