If you are referring to someone with a true Multiple Personality Disorder (MPD), it requires an answer from someone educated in this realm, either from experience or psychological studies. Would anyone of us know that the members were the same person? Would they have the ability to log in and off on the same site without knowing it?
If you are just referring to people that create multiple accounts on the same social site for the fun of it, for whatever reason other than MPD, so be it. I’ve chatted with a few that have done so, and I did it once on a different website. It was too high maintenance (and felt wrong), so that account was closed.
For those that shared their secret, I found it intriguing and funny. There was one person that created two accounts who let me in on his secret. He must have had two computers, because I could chat with both at the same time in a public forum.
On a side note, I appreciate when Milo chimes in on @gailcalled‘s posts, as well as when other Fluther members post the opinions of their SOs.