I don't think it is at all the same. MPD is a very specific diagnosis, if it does exist, those affected have huge gaps in their memory as they actually do not recall acts performed by other personalities. They also exhibit extreme disfunction in their lives, their issues make loving a normal life almost impossible.
I think the cult mind control of the JWs causes people to submerge their natural personality into one that is more acceptable to the organization. Get them away from the constant meetings and that natural personality will come out more, that is why they are so insistent about meeting attendance. But people do remember all events no matter which personality was more evident at the time, and they have normal lives, if you can call being a JW normal.