Okay, this is strange. According to >this post, from a reliable blog, Pessoa, a Portuguese poet, also wrote and published under several other names. These were not pseudonyms. He actually created lives, histories, and personalities for them. Or they created them for him, depending on whose view you take. This one man wrote, published, and publicized at least four different European authors. They sometimes had critical conversations in journals and magazines together. One criticized Pessoa's own work. Two admitted that their work was influenced by a third's. They had lives that were chronicled in public newspapers and magazines. But they were all just one guy.
Maybe we shouldn't try to cure multiple personalities. Maybe they'll win a Nobel or Pulitzer one of these days if we leave them alone.
Apparently it's true. I found other sites about the guy. Of course, they don't call his stuff “multiple personality disorder.” They call it heteronyms.