I have two 3 1/2 month old female kittens (sisters). Currently I am using only one litter box for both of them with no problems. I am constantly scooping, so the litter box is kept very clean. I change it twice a week. I am using Swheat Scoop which is great on keeping down odor. Is it really necessary that I add on another litter box so they each have one, and if so can the litter boxes be right next to one another?