J. Yo, Oakland, CA
Cats and dogs have unique and specific nutritional needs. Their respective foods are carefully tailored to meet those needs. Cat food, in general, is too rich for most dogs. Dog foods typically lack nutrients that cats need. Therefore, I do not recommend feeding dog food to cats (and vice versa).
That said, ice cream is rich and does not contain all of the nutrients needed by people. Yet we can still eat it in moderation, as long as our overall diet is balanced. And, in healthy animals, the same principle applies. As long as the overall diet is balanced, a little variation won’t cause much harm. So, if your pets are healthy and their stomachs aren’t upset by it, don’t worry if they occasionally nibble each others’ crunchies.