[Nanya01]

A thought came to me at work that Fate's apartment is where she really lives but Fate spends enough time at Nanoha's house that Nanoha has a guest room specifically for Fate to stay in.

Actually, it isn't that uncommon for someone to have a house/apartment of their own and spend a lot of time at someone else's house. My own mother, for example, has a house that she lives in with my younger brother, but she spends a lot of time out of town at a friend's house. So much so that she has her own room there, even though she doesn't live there.

So, I think of it like that then.