How we cite our quotes:
Tyler [...] signed Marla's name to the telegram order, and yelled, yes, Marla can be a guy's name sometimes, and the clerk could just mind his own business. (11.27)
And here's another joke with a point. (Who knew?) If our narrator can pretend to be two people, why not be called Marla? Also, with Marla and Tyler never in the same room, how weird would it be if Marla and Tyler turned out to be the same person? After all, "Marla Singer" and "Tyler Durden" have the same number of letters. Consider the implications of that. Are we reading too much into this?
Tyler's words coming out of my mouth. I used to be such a nice person. (12.44)
Yep, here's another clue that our narrator and Tyler are the same person. But it also shows us just how fractured our narrator's identity is. He distances himself from his own words, pretending that someone else is in control.
Marla hopped into the kitchen with both legs in one leg of her pantyhose and said, "Look, I'm a mermaid. [...] This isn't like when guys sit backward on the toilet and pretend it's a motorcycle. This is a genuine accident." (14.7-14.8)
So did our narrator accidentally become Tyler Durden, or is he actively doing it?