We have a similar situation at our house: Spike is on regular cat-food, but Oz is on weight management food. Oz used to be a stray, so when he sees food, he eats it (because in his mind, there might not be food later on); this means we have to have scheduled feedings in our house, because if we left out food for them to graze on all day, Oz would just eat it all at once and make himself sick. (We also feed our cats both wet and dry food, and of course the wet food can't be left out all day -- whatever they didn't eat would spoil, especially in this heat!)
I find scheduled feedings make portion control a lot easier. We measure out how much wet and dry food our cats eat, based on our vet's recommendations, and the cats know when they're going to be fed. (Cats have an incredible sense of time; even when the days start getting shorter, they still can tell when dinner-time is!) We have to stand over them and watch them eat, however, because they'll switch bowls partway through eating. Sometimes, too, we have to take Spike and his bowl into the bathroom, because Oz will just shove him out of the way and steal his food -- Spike is a very submissive cat, and if he thinks Oz wants to eat all of both their food, he'll just leave and go hungry, if we don't take matters into our own hands.
I admit, scheduled feeding times and having to watch the cats eat can be a little annoying at times -- the cats start crying for food about half an hour to an hour before it's time to eat, and I don't like interfering with them once they start eating -- but this is the easiest and most effective way to ensure both cats eat the food they're supposed to be eating.