Ooo. I never thought about it in terms of souls. I like "Moony" as a way for his friends to separate their mild-mannered friend from the beast he becomes. And I like Remus to think of the beast as a separate facet of his being, but not a separate entity. I always thought that Remus was always in there when he changed, but the nature of the curse was such that he reverted mentally to a very basic existence, full of primal urges that overrode his human side almost completely. If I recall, in canon Hagrid reported after Lupin's transformation that Lupin didn't remember "eating anything last night." So he has some recall of basic events, but not very much, and certainly no control. I love Lupin's situation because it's a metaphor for what all humans struggle with - our civilized vs. our animal natures, and we are not allowed to divorce ourselves from the beast entirely.