Thank you guys for all those ideas! Sorry for the late reply.
A few highlights on ideas mentioned here:
- @[email protected]'s idea of testing different litters might work. Frieda used to be a street cat, so she'd rather use dirt or grass (or finer litter), while Kika was always a home cat so she prefers more typical gravel-like litter.
- @[email protected] mentioned odour removers. I didn't try them, I used alcohol instead. Got to try it too.
- A lot of people mentioned smart systems that allow/deny entry for a cat based on the microchip. I gave those a check, and they're outright expensive here in Brazil. (Import taxes are specially harsh on electronics, plus cost of living is smaller than in Europe and CA/US so stuff like 100 euros or dollars is actually a big deal here.) I might want to try a poor man's version of that though, by not allowing Siegfrieda to access my bathroom at all, and placing Kika's litterbox there.
A relevant detail that I didn't mention is Kika's age - she's already 16, and cats get a bit stubborn when old (not that we humans are any different...). But I think that a mix of the solutions that you presented might work.