I only offer my cats one litter box (which I scoop out several times a day and totally clean and put new litter in every 2nd or 3rd day). They don't terribly mind, although they can always go outside in the garden once they are well-attached to the house and me. I only put a 2nd litter box when I had to isolate a sick cat in the bathroom, or a new cat in the computer room. I always want an indoor box available for when the weather is bad and the cats congregate in the house. In the winter I usually keep one in the bathroom and one in the computer room. Here, winter is only a few months, so it is no great hardship.
I think there is a lot of nonsence about cats and litter boxes. I use plastic laundry baskets and I clean them with a good detergent and then rub the inside with the leaves of the lemon geranium or with rosemary sprigs to make them more pleasant. I then sprinkle a very light covering of baking soda and put in the litter. The cats don't mind the geranium or rosemary, they don't mind the baking soda (which is a deoderant), and they don't mind sharing -- in fact I suspect that they rather enjoy placing their own :censor::censor::censor::censor: and piss on top of the other cats as a way of showing their superiority. I know there are some cats who feel they need their private toilet arrangements, and why not acommodate if you have the room? But a few minutes every few days to wash, dry (the sun does it here and drive oders away as well), and deoderize is less of a problem than all time, money and effort to buy the latest "effortless" litter box invention.
Put both litter boxes end to end somewhere for a few days, and then reduce the two to one. If one of the cats protests by messing on the floor, then you will have to provide two for a while again, but put them side by side or end to end until you've habituated them to accepting what is more convenient for you. You will find that you don't have to coerce the cats. Just have a little patience. They really don't like messing on the floor. They like to be able to dig holes and then cover their waste. It is an instinctive urge that helps to protect them against predators figuring out where they live. This is the same reason that they very often protest if their food and water or bedding are close to a litter box. In the primeval part of their brain, they know that a predator might locate their "toilet area" by smell, and that might lead them to the cat's sleeping place or the birthing nest. So keeping the food, water and bedding away from the litter box is much more important than worrying about having separate litter boxes. You may observe that many cats will preferentially use an area in the garden that all the other cats are using. When it becomes saturated according to their instincts, they will all then switch to another spot.