The kidneys don’t pump, so your question kind of goes away. Blood is brought in the kidneys by arterial blood flow, and the kidneys release blood into the circulation by passive flow into the veins. read more
Kidneys also seem to lose their ability to function at peak efficiency a lot faster than the rest of the body so having two of them allows you to live long enough for something more important to give out. read more
A function that never can rest. But with two kidneys, one can rest and clear itself of the accumulated waste while the other works. I believe someone did a theoretical model of the kidney function and noticed that when one kidney was removed the remaining kidney divided into two kidneys. read more