Is there competition for the essentials? Creating more sleeping areas may help. And i dont mean a line of beds!! Simon has a card board box under the table, its nice and secluded. They also have the bottom of my wardrobe, one section of a wall unit and the bottom corner under the bed where i leave a throw hanging over. Its important theres plenty of 'hot spots' for them share.
They also have a 3 tier scratching post that always has one snoozing onthe top level, a 2nd post with a hidey type base and beds near/on the radiator and all the human beds of course
The behaviourists rule is one per cat plus one, and thats not just for litter trays
Having a harmonious multicat household can take alot of effort but once you have the balance it makes it so much more pleasurable