[this is chuuya we're talking about why would you ask such a thing.

he folds his arms behind his head, thinking.]

All right. The thing with the hydra mention you guys found is that they don't exactly duplicate-- this isn't a situation where you're forming two demons when one fails to get exorcised. A demon who's still active on this side is kind of like a beacon to the others, and encourages them to join, which is how you end up with two when you let one go free instead of just keeping a single active demon. Keep letting it happen and you've got a whole demon party.

You let them go free by failing the exorcism, or by killing their host using normal means instead.