h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A good article, but your statement about always recognizing when personalities switch is not true. It is not always easy to tell when a change occurs, many times it is nearly impossible. For people with MPD to function in society they in fact usually want to hide their condition, so they learn to suffer through switches as quietly and subtley as possible. An alter personality will in fact sometimes mimic the dominant personality for this purpose or to fool other people who ARE aware of their condition.

But in order to mimick the 'normal' personality, the alter has to know about it -- and I thought the different alters don't know about each other? Or is it only the normal personality that doesn't know that "there's somebody else there", while the alters are aware of it and even each other?

The alters almost always know of the "normal" personality. They may or may not know of each other.
In addition, it may be a slight mischaracterization to say that MPS and schizophrenia have "nothing" to do with each other. Since the actual causes of each remain something of a mystery, we cannot say in good faith that they are entirely unrelated on a biochemical basis. It is true, however, that they typically occur in the manner you describe. Incidentally, it is unlikely that "schizophrenia" is actually one disorder; since we only diagnose it through symptoms. (For example, if you tell the doctor simply that you vomited, he can't really diagnose it without substantially greater information. We don't really have the kind of information we'd really need to differentiate causes of schizophrenia.) Genetics, viruses, drugs, and traumatic experiences are all serious contenders as possible causes of schizophrenia.

On a purely practical basis, suppose one has a mix of a dominant personality which is quiet and shy, and an alter which is loud and forceful, then the comments from society along the lines of:

"you're a bit loud today"
"this isn't like you at all"
"are you well?"

Would rapidly inform the alter of the desired behaviour to conform to societies overwhelming desire to pidgeonhole people.