There may have been a trauma of some sort which you are unaware of and which you may never know.

Cats rely on smell. I agree, you should always save some of his litter and put it in the clean box. Two boxes is best, one for each cat even if they both use the same one. It just takes away the pressure.

Some folks I know have used Bach's "Five Flower" or "Rescue Remedy" herbal tincture and put it in the water. Good for anxiety. I know some people who have had good results with it with anxiety.

I "free feed" my 4 mogs. They seem fine. I have 5 litter boxes for 4 kitties. They are strictly indoor kitties.

Have your habits changed, i.e., are you away from the house more than usual? Sometimes kitties are just missing us and it comes up in these ways. If the medical health is good, sometimes it's just time that heals the anxiousness.

The kitties should not feel they have to compete for affection.

I don't know if my ramble helps, but I hope so Your cats are beautiful.