I think the first beginning of any split, the first seed planted, was back at the beginning of the first grade, inadvertently, by my parents.
All my short life, I'd been Joanie, at home, unless I was in trouble, in which case I was Joan, or, in extreme cases, the full set of names.
My mother enrolled me at school as Joan, as it is, of course, my legal name. So, from day one of first grade, I was Joan at school, and still Joanie at home. Joanie has always been my name.
I did not have close friends in my classes. First grade, I was sort-of-friends with the popular girl, Colette (the one who later asked if my parents were divorcing), and later became friends with Ruby (and Jay), though there was intervening bullying. Sarah was a year older, and my friend. Ginger came to my school in 2nd grade, but left the next year. Narcissa was in first grade in my third grade year.
I had no close friends in any class. I barely talked to anyone in my classes. 4th grade was the loneliest year, where I started to notice the differences between me and everyone else, sharply, where I began to get taunted the most. Sarah M. was friends with me that year, because her friend K. wasn't there that year. Calico was my best friend that year. I spent recesses with the other Sarah.
5th grade was a reprieve. Ginger, Gaia, and Galadriel were all in my class, and we were awesome. We were good friends. Krissy was kinda with us, but kinda not. This was the first year I'd actually had friends in class with me, and it felt good.
6th grade, I resolved to become the appearance of cool. Didn't really work. Ginger left, and our quartet fell apart without her. Gaia and Galadriel were friends, I was friends with Galadriel... and it had worked when Gaia and Ginger, and Ginger and I, closed up the circle, but it fell apart. I had a boyfriend anyway, and I got to know Michelle. She was awesome. (She was on the geeky side of popular/cool, and later became one of the high school's neohippies.)
By 7th grade, I was feeling lost again. Our little group had merged with a new set in middle school, and I was on the outskirts. Savil was the center of the group, of course. I was friends with her, but losing touch with Michelle and Galadriel, and never having had touch with Gaia.
When the substitute reading teacher (Mr. Addington, who I took a liking to after hating him for replacing Mr. Murphy) left a book about Eve where I could get at it, I read it, fascinated. She was able to do what, again? Interesting. There were already several prominent facets of me. Ginger had insisted on spelling my name Joni. I was Joanie at home. I was Joan at school. Other people felt that way inside too. Only they didn't have very good control over it. I had to be able to keep it all together. So I could be Joan at school, Joanie at home, and Joni when I was happy and laughing and accepted by my friends. That was fair. I could do that.
The first really really visible sign of multiples came in 9th grade. 8th had been a holding pattern, and when my depression kicked in. I didn't really fight it. I just sort of rode with it, trying to ride it out, not knowing what was wrong, hiding in a succession of sf/fantasy books. (Bless you, Mr. Murphy, for leaving Menolly where I could meet her.)
9th grade saw me grimly hanging on, lonely, with no one close but Savil, and my books. I had escalated my journal-writing, as it was something I could do undetectably in class. It would look like taking notes. I'd perfected that back in the 8th grade. Imagine my surprise when I wrote back to myself, from a different mental perspective -- with a different handwriting.
There were two of us, at first. Me, and Shanna.
Then there was the Vulcan. And then the Vulcan and Shanna merged, and then there was Laughing Hawk. And Mona was in there. Quiet Mona.
Shanna never let me know how depressed I was. When I'd get bad, we'd remind us that we had to hold on to see the new Star Trek movie coming out in the winter. That got us through the fall. Then I had CTY to look forward to.
...The rest is history.