Throughout history many intellectuals and spiritualists have tried to define how it is possible for one person to take on multiple personalities and also to be accurate with the information of those personalities. Such is the story of Lurancy Vennum. Full information can be had about her story by visiting: https://www.mysteriouspeople.com/Lurancy_Vennum.htm
"She awoke to find bandages covering her eyes to protect them from her unconscious scratching; but instead of removing them, she discovered that she seemed to be able to see as easily while blindfolded as she had before. Family friends, including A.J. Smith, editor of the Danville Times, and the Reverend J.H. Rhea, witnessed Mary Roff, heavily blindfolded, accurately "read" to them the contents of a sealed letter in the editor's pocket, and arrange, correctly, a pile of old letters which she could not see. The amazed editor wrote a long, detailed account of the incidence in his paper."
Author. Brian A. Haughto
This didnt seem to be a case of multiple personalities as it was a case of possession. My definition of someone with a multiple personality would be that they have created their own personalities and are living these out on a daily basis. These cases seem to be linked with the person being possessed by others, who are living through them. It is indeed strange though how these possessions come to be; I have always felt that there was a weak character trait that would allow such control to take place but then again, that is just an assumption. Very interesting story indeed!
In general from my theory of cosmic reincarnation, people can have multiple souls. This can occur during wars when the amount of deaths is much larger than the amount of births. This necessitates some souls to coexist within the same body. Each soul has its own mind and history and this gives the person multiple personality disorders.
When more than one soul tries to occupy a body then one of those souls is very selfish. This means one is likely a bad spirit and needs to be expunged.