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The situation is this : Dogs can eat cat food and this is no problem and it will sustain a healthy life. However the reverse is not true, dog food is not nutritionally suitable for cats as they have different dietary requirements. So you should try to feed only cat food to Tammy if possible, you may have to experiment with various brands.
That said if say two thirds of Tammy's diet was a commercial cat biscuit such as Whiskas and one third was dog food then this would be probably be near enough nutritionally balanced for cats.