No one would offer both kidneys. That, too, would be immoral. Because you are sacrificing yourself. You are putting the value of another's life ahead of yours. This is altruism.
Few people live by pure altruism.
But many preach it. They do not know what they ask. It is immoral ground. Yet, it is argued in the case of objectivism. We are quick to dismiss ideas with which we find any flaw whatsoever, choosing instead to live in a moral limbo.