
Convert your feeding style from a shared free choice food bowl to feeding 2 or 3 times a day in MEALS. Once the meal is done, the food goes away. This discourages the cat that likes to overeat when there is nothing else to do. It also allows you to feed the cats in separate rooms with a door between the fat cat and the skinny cat. With this plan, you can feed each cat an appropriate food for their size. This will also allow skinny cat to eat at leisure and fat cat to cool his heels after the meal is gone.
If you are concerned that skinny cat needs food out all the time to graze on, then try placing skinny cats food in an elevated location -like the top of the refrigerator- where fat cat can not physically access.
Or, you can "build" a feeding box that has a skinny cat silhouette as a door, so fat cat can not enter. This allows skinny cat to go in and out of the feeding box to graze whenever he wants.
DIY Cat Feeding Box

There are some really COOL interactive feeding toys available for purchase. This will challenge your cat, give them more of a hunting experience, and slow their eating down -all of which are beneficial to both skinny cat and fat cat.
Select a low-carbohydrate food for weight loss, as a high protein food will increase energy expenditure. (i.e.-carbs make cats sleepy, protein makes them more energetic). See my previous blog on the importance of feeding high protein foods to cats: Be Savy about your Cats Nutrition

As always, thanks for sharing this information with your cat loving friends. Dr. Maureen Flatley
Fox Valley Cat Clinic