I have the same problem. My vet told me to change meal schedules, and to try to be there when I feed them. Of course, my two cats love each other's food, but when I say "no no" they change bowls! Since the youngest cat has arrived, Grisouille, the overweight one, runs more and plays a lot, so she in fact lost weight even if she - maybe - eats more. If you can be there 4 or 5 times a day, it should be enough. If you can't, try to make them understand what is best . Unless the youngest can go places where the older one can't. Good luck!