While there are a few canned formulas available that meet the needs of both species, most foods are designed specifically for cats or dogs.
Animal Nutritionals has investigated this enigme!
Cats require a higher percentage of protein and fat than most dogs and they have specific requirements for additional taurine. Dogs that eat too much cat food are at risk of weight gain and even pancreatitis. Cats that eat dog food are at risk of weight gain when the food is high in carbohydrates, as well as more likely to develop deficiencies in important amino acids like taurine. Read more in this amazing article 🙂