It is just instinct for cats to go in dirt. I had a cat that used a large potted plant at times! That was annoying. The one cat may have decided she doesn't like sharing a litter box, you may have to get two of them. Maybe put one out on the patio, it would be up to you how to keep them separate for each cat... I never had to do that my two cats didn't mind sharing. If the cat is pooing on the patio, take the poo (this should happen right as she is done) don't scold her, just take the poo and take her with you and put the poo in the litter box and show her that is where it goes. It may take several tries... Good luck!

 The last answer was very good advice. Natural instinct or the sharing of the two boxes may be a problem for your cat. Also, though, if the cat has ever been an outside cat, they will do that. I've seen strays do it many times. Its just a cat thing.