I got a weird problem involving both of my cats (Siegfrieda, to the left; Kika, to the right).
Kika is rather particular about having her own litterbox(es), and refuses to use a litterbox shared by another cat. Frieda on the other hand is adept to the "if I fits, I sits, I shits" philosophy, and is totally OK sharing litterboxes.
That creates a problem: no matter if properly and regularly cleaned, the only one using litterboxes here is Frieda. We had, like, five of them at once; and Kika would still rather do her business on the patio.
How do I either teach Kika "it's fine to share a litterbox", or teach Siegfrieda "that's Kika's litterbox, leave it alone"?
Try training Kika to use the toilet? My Grandmother's cat was the same way back in the 80's, and my uncle (yup, he still lived at home in his forties) trained the cat to use their downstairs toilet. Problem solved.
Edit: we kids always tried to catch her doing it but never did, but wed see the evidence after the fact.