We deal with characters sealed inside of them similar to how we deal with bringing in pets. If they are appable on their own and characters in their own right, then characters will find that their bodily co-habitant is no longer with them. Nothing but tumbleweeds on their side of the head space. Complete with accompanying fwooooo wind sounds and everything.

When considering what I picked up about Kyuubi from osmosis, in Mito's case, regardless of canon point, Kyuubi is a potentially appable character and thus, would no longer inhabit her body.

Hope that helps!