Litterboxes are places where proper cats can 'mark.' There is not such thing as " your box" and "my box." THey are all community boxes.

I have 8 boxes and at times, only 4 have been used in a 24 hour cycle. This happens in particular if I added fresh litter to some and not others. Everyone has to "christen" the new litter and make it 'theirs.' So I wouldn't worry about that one bit.

As for feeding, feed them separtely behind closed doors. I have at times used 4 bedrooms, one bathroom, and the hallway to get everyone their proper diets. If you are afraid you will 'forget' someone, set the kitchen timer for 10 minutes. I have done that when things get hectic around here, as well.

Best wishes!