Make sure the stronger cat is not keeping the weaker from the litter box. (Blocking access to it is a sure way to make the victim miserable.)

Have two boxes on different floors, if possible.

(Did you ever read "A Tree Grows in Brooklyn"? I remember the narrator telling how mean boys in school would block access to the toilets for some strange reason. When I learned that cats block access to the litter box, I decided those bizarre boys had merely reached mammal status, having been denied humanity by some circumstance of poverty or abandonment.)