How does Hill's Prescription Diet k/d for Cats help a cat with kidney failure?

Our vet has recommended that our 16 year old cat be put on k/d, saying that he's starting to show signs of kidney failure. How does this food work to help his condition? We have another elderly cat, age 17, who is not experiencing kidney failure. Would it be harmful to her to ingest the other cat's k/d food? I doubt I would be able to completely segregate them from getting into each other's food, so we'd like to just switch both of them over. Any suggestions are appreciated. Thanks!