Separating overweight and normal weight cats during feeding time is one solution to ensure all cats get their required daily caloric intake (and not too much). This may require adapting the cats to a feeding schedule rather than allowing free feeding. If the cats are home alone during the day or if they are fed during the night, this approach will require keeping separate litterboxes in the rooms. Also, switching from premium diets to standard-type products may reduce the caloric intake for your cat. If these solutions don't work, try changing to a "light" product for all of your cats. Consult your veterinarian for advice.
For multicat households where the cats have different tastes, it is important to keep in mind that a diet containing lots of fruit and vegetables might be fine for humans, but such a regimen is not good for cats. Cats are carnivores and need dietary sources of nutrients such as taurine, vitamin A and arachidonic acid. Each cat in your household should be fed a complete and balanced diet with an occasional treat or two. Research has shown that animals do not eat to satisfy their nutrient needs, but rather their likes and dislikes, so be sure to find a complete and balanced cat food your cat likes.