Can a character be in two packs at once?

Ah, characters can only be in one pack at a time, for a few reasons, chiefly the fact that the system was designed to only allow one pack at a time and it's not actually possible to be ranked and counted in two. It's also for fairness. If she's in Larksong Grotto then she isn't in Rosings and and vice versa, so she shouldn't count for two packs.

You are welcome to play this out ICly by either having Hosannah titled in Rosings and hanging out in Larksong Grotto (but she would not be a part of the pack nor could she help lead it in any capacity), or having Hosannah titled in Larksong Grotto and reporting to Rosings, but either way she is ranked and listed in only one of them. If you decide to go this route, keep in mind that she is technically a trespasser in whichever pack she isn't a part of—if a member of the pack decides to attack her in their territory for whatever reason, then a severe injury is still mandatory even though she's playing double agent.