Two cats, two different diets, two different eating habits.

My cat and I recently moved in with some old friends who have a cat of their own. My cat is a lean cat with special dietary needs, and eats a special food for bladder health. Their cat is a tubby fellow who will eat anything and everything to the point of vomiting. We feed the cats in separate rooms, but my cat's food is more filling, and my cat munches on 1/2 a cup of food through out the day. Their cat eats 1/2 cup twice a day, and eats his portions in one sitting. Any time he comes into my room, he tries to eat my cat's food, since my cat doesn't eat it all at once. I'm not sure what to do. Even feeding my cat 1/4 a cup twice a day still means he'll leave about half of his serving behind, which will promptly get gobbled up by their cat.

These cats love to play with each other, and I'd hate to keep my cat completely cooped up the entire time. My cat just doesn't eat fast enough. He's very thin as it is, and I don't want him losing any more weight.

Any ideas?

Edit: "My can and I"