A while back I had an elderly cat that I fostered for the RSPCA, due to bowel problems she was on a strict prescription diet which was a nightmare with my existing cat as they used to eat each other's food!!

It wasn't too big an issue if my own cat ate the prescription food other than the fact that it was expensive and the RSPCA were paying for it but the elderly cat got very ill if she ate any other food.

I solved the problem by introducing set feeding times, e.g once in the morning and once in the evening.

At feeding times I would fill a bowl of each and place the appropriate cat along with the appropriate food in a closed room until they ate there food (only a couple of minutes as they tend to finish all the food when on a feeding time).

I then took the bowls away until the next mealtime.
This worked well and ensured that each cat got their own food.

Hope you manage to sort something out!