A CRF cat should be on a low Phosphorus diet, whereas a diabetic cat:
Feed your cat wet food.
Feeding your cat non-dry cat food is the best thing you can do for your cat, diabetic or not. A cat is an "obligate carnivore." That means the physiology and anatomy of the cat are suited to eating a high protein, meat diet. Dry food, even prescription diabetic food, just cannot meet this requirement.
A low-carbohydrate, high protein, wet diet has been associated with remission (disappearance of symptoms) of diabetes in cats. You can feed your cat a raw or cooked diet that you make yourself or there are many excellent high protein canned foods available. As a cost-conscious compromise, you can feed readily available low-cost canned foods, such as certain varieties of Fancy Feast.
Do your cat a favor and feed her protein.
So what to buy that wont harm both of them?????