@Cheshire
In what literature there is on multiple personality disorder (which is a pretty contentious topic in itself), some of the subordinate personalities do display knowledge of the other personalities and the "primary" personality, even to the extent of plotting against them. As with many psychiatric disorders, it's really hard to say much for certain, though.
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As for the story, I enjoyed it a lot. The repetitive, rhythmic phrasing reminded me a lot of "Red Rubber Gloves."
I have to say that I was mildly surprised that the multiple personality was a surprise. Given the absence of backstory hooks, that was pretty much the only option, and the little rant about reading and hearing it in his voice cemented it as the ending early on. The narrator's manner of speech made me irritated and uncomfortable; it was a pitch-perfect rendition of how actual mentally ill people talk, repetition and odd derailings and all. (I have to talk to actual mentally ill people periodically due to the nature of my day job. It's rarely a pleasant experience, but I do have a really solid handle on what anxiety, depression, and schizoprenia sound like nowadays.)
It was a good story, though as ever Alasdair's post-text analysis added interesting depths and additional layers that made it better. In particular, Al really highlighted Dala in an intriguing way.