For how long has this gone on? This could be medical.  Have you taken your cat in to check for a UTI?  If not, this should be step 1.  Take a fresh urine sample with you, if possible. This could also be behavioral.  Is your cat stressed?  Do you have two or more litterboxes (one per cat plus one), and does your cat use any of them?  Have you tried different litter types?  Has your schedule changed suddenly, or is someone new in your life?  Could your cat be bored?  Do you use room sprays and scents that might bother… Read more »