I've been struggling with the same problem, and of course the one who's not on the special diet just has to have the special food. My vet told me it wouldn't hurt him, though.
The best solution is probably to put them on a feeding schedule, twice a day at the same times every day. Either feed them in separate rooms or supervise their eating.
Of course, my advice is "do as I say, not as I do," because I have such a screwy schedule that I'm never home at the same time to feed them. I just leave the food out.
I will say that after six months of me steering them to the right bowl whenever they ate, they may be starting to figure it out.
Good luck
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