It is possible for one alter to have MI and the host / other alters not.

Check out the DID forum if you want the inside scoop.

That being said, it is possible for psychosis to fragment your mind in a sense. I was once 4 people during an episode, but I don't have DID, just came very close to unravelling Also dissociation as mentioned above by dear Dark Heart, can cause a type of amnesia while in that mind state. Think blank rage.... I've been hearing since childhood that I'm capable of some violent acts and stronger than people twice my size when enraged. But I wouldn't know