I think also it helps if we don't think of each human person being separately created. In actuality, each human person is generated from his parents, i.e. he shares the same nature in its entirety with his parents. God in fact only ever created one human being, Adam, and miraculously generated Eve from Adam. All other humans have been generated from Adam and Eve by a natural process of reproduction, but God has never specially created a new human being. So the human nature that fell with Adam and Eve therefore determined the fall of all subsequent humans.
One could object that our souls are specially created, but in fact the Fathers are not agreed on this; some say that our souls are also inherited from our parents (sorry, I don't have the quotations with me right now, bear with me). What is true is that, mysteriously, each product of this natural process of reproduction is hypostatically unique, with a unique person, body and soul, and with a unique will. The unity of human nature does not contradict the diversity of hypostases, just as the unity of the Divine nature does not contradict the diversity of the Trinity.