Wrong cat getting into k/d dry and thyroid dry

Twice now, my diabetic cat has gotten into Cocheezie's k/d and thyroid. Yesterday, we came home to an empty dish (not Cocheezie), and today I heard crunching and ran. He's known about Cocheezie's food on top of the cedar chest for months. Why now?

Questions:

Will the food harm him? This is the most important question. There is never much in the dish, usually less than 1/4 cup. She's gone off k/d wet and will only eat wet that has no k/d mixed in, so I'm leaving some dry out all the time now. It is important to try to maintain her weight.

How do I discourage him?  Gentle and repetitive removal hasn't worked. I don't want to lock Cocheezie in one room (even though she seldom leaves it). There is no other possible place in the room that Cocheezie can access.

Any suggestions?

I dont know what thyroid food is. But when one of my cats was on diabetic food. They all ate it. It was just easier. The vet said it was fine. Sounds like you need to feed them seperatly. Do you leave food out for them or schedule feedings?

Thank you. Diabetic cat is on NV Instinct dry + FF wet + some Merricks and Wild Calling.

Cocheezie is on k/d wet (I wish), Wild Calling + some Merricks + a tsp of forbidden fishy cat food twice a day mixed with 3 meds. Her dry is k/c and y/d and she doesn't get or eat much of it. I don't like her on the dry but that is the only way she will eat the RX food these days. The weather is getting too hot to leave wet food out for any length of time.

Both have food left out all the time. Diabetic cat borders on pica and will eat inedible objects off the floor if no food is available, and Cocheezie is terminally ill and needs to have access to food for whenever she feels like eating.

I just remembered that the diabetic cat's previous owners used to shake a partially filled water bottle at him when he was doing something he wasn't supposed to. Going to try shaking a water bottle at him whenever he approaches her food. (No spraying of water - only shaking.) And will shut the door to her room when we go out.

The question remains, however. Should a diabetic cat have k/d and y/d?

Have you talked to your vet to try to find a food both cats can have? So you wont have this issue?

And if i was you I would put the cats in different rooms for now tell you figure something out