Do you all remember Porch (the one who had megacolon and most of his colon removeed)? He has to be on a vpenpery strict diet, but luckily for us it's OK for the other cats to eat it. It's just VERY expensive, but worth the price over the hassle.

You might try switching the back and forth in the room so the other one thinks she's also getting special treatment. It also may settle down once they get use to the routine.