The first thing you need to do is take them to the vet to rule out a Urinary Tract Infection. Cats usually don't do what yours are doing unless there's a problem

One rule of thumb is one litterbox PER CAT. I would advise getting another litterbox.

Get them fixed ASAP. That will also stop the improper urinating. You're also going to need to get a black light and search for the spots they've been peeing in. This way you can get rid of the odor so they don't continue.

Good luck.