That's a good argument, but there's one important flaw in it.
Norton-Tyler saw himself as a seperate entity from Pitt-Tyler.
Norton-Tyler did create Pitt-Tyler as a defense mechanism, it's true. The existence of Pitt-Tyler was solely to justify Norton-Tyler's breakdown by "making it someone else's fault," as it were.
However, Norton-Tyler didn't consciously create Pitt-Tyler with that intention. So far as his conscious mind went, this was all new and confusing stuff. Furthermore, when called by the name "Tyler Durten" he became massively confused - suggesting that this was not, in fact, Norton-Tyler's real name.
What this gets into is the real nitty-gritty of existential crisis: "If I don't think I exist, do I really exist?" Norton-Tyler believes that he exists as a seperate entity to Pitt-Tyler. Furthermore, he was the original persona - Pitt-Tyler didn't create *him*. So, since he's the original and he believes that he is a seperate entity... well, for all intents and purposes within their shared head, he IS a seperate entity. That's the very basis of Multiple Personality Disorder - two or more completely seperate consciousnesses co-existing within the one mind.
Leading up to Project Mayhem's great plot, Norton-Tyler began to realise the truth about his relationship with Pitt-Tyler, and react to it. It quickly became obvious that he couldn't simply reject Pitt-Tyler any more; that persona had become far too powerful to be killed.
This is what I meant by Pitt-Tyler being the dominant persona. He was aware of their split-personality nature, and being aware of it allowed him to access Norton-Tyler's mind and forcibly take control of their body.
However, Norton-Tyler began to figure out how to do that for himself, as shown when he deactivated the bomb in the back of the van. By the time of the climax, Norton-Tyler accepted what had happened in his life and began to regain his role as the dominant persona. At this instant, he accepted who he was - and consquently, Pitt-Tyler was no longer needed. The defence mechanism wasn't necessary any more, which was what allowed Norton-Tyler to "kill" Pitt-Tyler and re-merge their minds.