Cats with opposite diets

Two of my cats are on opposite sides of nutritional needs. One, an 18 year old who by all means is perfectly healthy (although verging on KD), is showing weight / muscle loss and want to make sure she is eating when she wants. My other cat, a 3 year old who was spayed back in January has put on considerable weight over the last two months and we need to start being more restrictive with her food. Then in the middle, I have two other cats who are perfectly fine weight wise and could likely go either way.

Short of isolating (or dividing) them up, any best practices for keeping everyone fed appropriately?