My cat is a dog and my dog is a cat?

I have 2 pets, a Pomeranian names Roxy who is a VERY picky eater she's a year old, and a 30 pound cat named Marbles and he's 9 years old, They both eat each others food. The cat eats the dogs, and the dog eats the cat's (if there's any left)...I read its bad for cats to eat dog food and vice versa..how can I stop these behaviors..? Oh and if any of you have tips on a very picky eater dog wise, those would grealy help.. :D