[quote=@Legend]
<Snipped quote by Yatagarasu>

Shion could always have multiple souls. If your bodies were barely compatible, maybe your souls weren’t. Or the souls could have come from somewhere else.
[/quote]

*Nods*
Correct. I believe that's close to the answer, so the question is... Why the major difference between them? If it's a mixture of, well, not quite physical per se, but close to it, from your mother, and Shion's soul being unstable, then why does splitting bring out the other version when there's no trace of her in their combined state?