Is a kidney transplant necessary to save the other kidney?

No: The goal of a kidney transplant isn't to save the other kidney because usually both kidneys have failed if you need a transplant. A single kidney transplant takes over the function of both failing kidneys.

No: I am not exactly sure of your question but you do not need a kidney transplant unless both of your kidneys are not working. As long as you have a working kidney, even if it is only one. You don't need a transplant.

Purpose of Renal Tx: When a person has renal failure, it is not just one kidney but both kidneys that have inadequate function. Therefore, a new kidney is needed to assume enough kidney function so that excess electrolytes (potassium, phosphorus) and fluids can be excreted.