There is always a reason for cats doing this. Well......if the second box is not an option your cat may never stop doing it. The golden rule is one cat litter tray per cat plus one. Ideally you should have three in different parts/rooms. Have anyone noticed - most cats doing No 1 in one litter tray and No 2 - in another?

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"Do not meddle in the affairs of cats, for they are subtle and will pee on your computer." --Bruce Graham
There are no rules, rules are only for people who fit inside a box, if you want to think out of the box there is no hope... Life has no Box. (Tony Howells)