I seem to recall from The Three Faces of Eve (?--the autobiography upon which the movie Sybil was based) that a number of her personalities were both cognizant of each other and interacted, though likely through a more internal/direct method of communication than internal or vocal speech. Other personalities were more isolated. This accords with other reading I've done on MPD and on brain function in general. We all have multiple personalities, to an extent, which kick in according to context and circumstances, though the barriers between them are generally weak enough that we mostly perceive ourselves as having only one. Short of spirit possession.

Didn't Bill Maher's film Religulous include a clip of some religious leader "speaking in tongues"? It was also unconvincing, but given the venue, what else would one expect?