Why the theft matters in both directions
A prescription diet is dosed nutrition: it is formulated to manage the patient's specific condition, whether that is urinary crystals, kidney strain, a food allergy or weight loss. Every stolen mouthful harms twice. The patient loses part of a meal calculated for her needs and may fill up later on the other cat's ordinary food, while the thief eats a restricted diet she does not need — some prescription formulas are deliberately low in nutrients that a healthy cat, especially a growing one, should not be restricted on long term. Casual grazing makes both cats' diets wrong.
Separate the meals physically, not hopefully
Wishing and watching do not survive a turned back, so change the geography. Feed the cats in different rooms with the doors actually shut, and stay through the meal — most cats finish within fifteen to twenty minutes. The patient eats her prescription portion in peace, at her own pace, and the thief gets his own correct food where he cannot trade up. When each cat is done or walks away, lift both bowls immediately; a prescription meal left down for later nibbling is a meal shared with whoever finds it first.
Close the gaps between meals
Most theft happens outside mealtimes. Store the prescription bag or cans in a closed cupboard or latched container, never in an open bag on the floor that a determined cat can tear or tip. If either cat is free-fed, that arrangement has to end for both, because dry prescription kibble standing in a bowl all day cannot be supervised. Treats need the same discipline: the patient should receive only what her diet allows, and her treats should not be dispensed from the communal jar by a kind-hearted family member who has not heard the rules.
Technology for the hours you are out
If your schedule means some meals happen without you, a selective feeder removes the temptation entirely. These feeders stay sealed until the assigned cat's microchip or collar tag is read at the dish, so the patient can eat when hungry while her prescription food remains locked away from her housemate. They are especially useful for a slow, grazing patient whose food previously had to stand down all day. Introduce the feeder with ordinary meals first so she trusts the device before her health depends on it.
Why feeding each cat on a different floor or time fails
Half-measures collapse against a motivated thief. Feeding the patient an hour earlier just teaches the other cat the new schedule, a bowl in a high place is an invitation to an athletic cat, and shutting the thief in one room while leaving the prescription bowl down afterwards still exposes it the moment the door opens. The only arrangements that hold are ones where the food itself is physically unavailable once the supervised meal ends — lifted, stored behind a latch, or sealed inside a feeder that recognises only the patient.
Loop your vet into what has been happening
Tell the clinic about the sharing at the next check, or sooner if the patient's signs flare. The vet may want to recheck the values the diet is managing, because a month of half-eaten prescription meals and stolen ordinary food can explain a disappointing test result that would otherwise look like the diet failing. Once your new routine is running, note it clearly so everyone in the home follows it identically, and let KinsLe Care Teams hold those separate feeding, bowl-lifting and storage duties, so your patient's prescription meals stay hers alone whoever is on feeding duty.