Here my kids used to poo twice a day just after meals and now they go just once - at the moment they want.
And one of them stopped covering the stuff - sort of " mom Loreena will cover or you will scoop,so, why I should I do the work ?" :-)
No litter sand or food changes .Poo is ok and the vet say theres no worms if so .I see none this time but I was wondering some could cause obstipation ( but they keep eating ok )
There doesn't really have to be anything "big" that changes as long as the change is in their litterbox.
A change in litter, a new cleaning solution used when changing litter, a lingering smell of the cleaning solution after litter change, etc, etc, etc,..
Also, any change to their food can cause a reaction, and if they're both able to go outside they can both pick up the same disease at justabout the same time too.
So they both stopped using it at the same time? Are they related? Both the same sex? Spayed or neutered? Have clean litter? Maybe you could try one for each kitty and have you changed litter brands? Oh, and once they've "gone" on a certain spot on your carpet they will return there if there is any residue of scent still on it. Can you confine them to a room with their litter and food, just for awhile so they'll get used to using the box(es) again. Good luck and kitty hugs to you all ((>^.^<)).