Is it safe to mix a little cat food in with dog food?

I have a medium size English Shepherd who is a very picky eater. I feed him Moist N Meaty which he wont touch unless I put a little cat food on top. We own a cat as well, and the two of them get fed at the same time. I understand the nutritional values of either foods which is why I want to make sure that a spoonful with each meal wont hurt him.