I would talk to your vet. Cats are complete carnivores, with only a small need for a small amount of green plants occasionally, they need a very high protein diet. Dogs are "opportunivores" who hunt meat as well. They eat mainly meat, but also incorporate fruit, vegetables and some grasses into their diet. A diet too high in protein (like prepared cat food) will eventually harm the kidneys of dogs, due to attempting to process the excess protein. Dog food isn't high enough in protein for a cat's well being.
I would talk to your vet and until then, keep their food separate.