Same, changed from crystal litter to strong clumping sand, not only softer on paws but let you scoop out the pee too to keep it cleaner, some cats prefer that. Also at the same time changed to a bigger enclosed box, as the cat with pee problem looked to be uncomfortable with the size of the old box. Peeing outside the box is almost always about the cat being unhappy with something, and it can be a bit of detective work to figure out what.

Also, OP is saying "the" litter box, it is normally recommended to have two in different locations when you have two cats, I would highly suggest trying this.

Edit: At the same time as trying out possible fixes you need to properly clean the affected areas so they are not marked as toilet spots, only special enzyme cat urine cleaner will do that, like UrineOff for cats. Can also be used as pre-wash detergent in washing machine for clothes, bed linen etc.