I agree with separating the two when it's feeding time. I have four cats, one of which is on a special diet. Every night I feed him in a separate room from the others. My cats all devour their food so everyone is done within a couple of minutes. Then I can let him out. All of their food gets measured out. The other 3 I feed science diet light, and they have ALL lost weight!
It's not that hard to separate a cat, it's not that time consuming and it's better for the kitten-poor thing needs to eat his share too!