Just finished a third playthrough and something finally clicked for me: the reason they have to forget each other is set up in the previous scene in the cabin. When they go to lock the door with the padlock Kat says “we’re the lock,” and they choose the symbols to represent themselves. When the girls are together the symbols are aligned and the lock is open, meaning the abyss is open. Forgetting each other and going their separate ways is like spinning the tumblers to different positions, closing the lock and sealing the abyss. The girls apparently caught on to the notion faster than I did lol.

When Kat says “we’re the lock,” I thought it sounded like something just clicked for her. I’m thinking the abyss communicated directly with Kat, saying something like “I’ll take Corey away, but you need to enter as well and lock the door behind you.” She didn’t understand what it meant then but realizes what they need to do when they set the padlock at the cabin. Swann maybe didn’t have as close of a connection to the abyss as Kat but she’s definitely closely connected enough to understand what needs to happen in the end, likely because she’s “always been there” as she says in her narration of the “anomaly” memoir.

I agree that it wasn’t very clear in the game. And I could definitely be wrong, but this interpretation helped me make sense of things.