One voice in a head is enough, isn't it?

In a lengthy Atlantic article about research comparing psychics who claim to hear voices of the dead, and schizophrenics who also hear voices, there is one paragraph about an idea ("tulpas") which is new to me:

In her work, Luhrmann has come across groups of people who—unlike Jessica—hear voices only as a result of practice. She gives the example of tulpamancers: people who create tulpas, which are believed to be other beings or personalities that co-exist along inside a person’s mind along with their own. “Somebody in that community estimated to me that one-fifth of the community had frequent voice hearing experiences with their tulpas, that their tulpas talked in a way that was auditory or quasi auditory,” Luhrmann said, a practice that she was told takes two hours a day to develop.“That’s connected to work. Psychosis is not connected to effort. It happens to people.”

Odd.