Newton's law of cooling is based on the observed fact that heat flow is proportional to temperature difference. So if you start off with twice the temperature difference you get twice the heat flow. When the temperature of the "hot" water reaches the starting temperature of the "cold" water, it must continue following that law and follow exactly the same temperature profile that the "cold"water followed.I've said this in a previous thread, explain why. It does no help to just state something without providing an explanation to the statement. By explaining your statement we can get to the answer of this thread quicker and more efficiently.