Fight Club Film Theory: Tyler Durden Isn't The Only Character Who Doesn't Exist
3. Marla & Tyler
There are some very obvious tells early on that something is up with Marla and Tyler, namely that Tyler tells Jack not to talk about him to Marla, and the two don't really directly interact outside of their sexual escapades.
Jack even compares this to the surreptitious behaviour of his own parents, and it's an apt comparison as Marla and Tyler are effectively parental proxies - two sides of Jack's psyche duking it out for control.
Even the sex scene between the two is basically a CGI fever dream where the two bodies are fused together in a fantastical state. With our theory in mind, this would actually be an act of masturbation, tying in with the many, many references to masturbation made throughout the film (namely, "Self improvement is masturbation.")
There's also plenty more to consider from a non-sexual context: the speed with which Tyler gets to Marla's apartment to prevent her suicide, for instance, could imply he never really left, being the same person and all.
Plus, Tyler saving Marla to save himself would also tie up the nagging question raised by Jack - "How could Tyler, of all people, think it was a bad thing that Marla Singer was about to die?" It doesn't really vibe with his nihilistic mindset, unless he's doing it out of self-preservation.
Sure, the mechanics of Jack basically preventing his own suicide is a big, wide-open can of worms logically-speaking, but again, it ties into the overarching idea of Jack's/Tyler's/Marla's narrative being unreliable.
It also presents an interesting paradox, that Marla and Tyler need to try and co-exist out of fear that destroying the other will destroy the whole - no matter that the end of the film proves Jack's ability to "kill" one of the personalities under the right circumstances.
But make no mistake, Marla and Tyler are firmly opposed throughout the film in an existential battle for Jack's soul...