Answer: They had to use two different bombs because the first one was destroyed at Hiroshima
but it also could it have something to do with the fact that Germany had unconditionally surrendered in early May 1945 and Japan didn't agree to surrender until August 1945, less than week after the second bomb was dropped.
Most people who are familiar with the events at Dresden in early 1945 will not agree that Germany was spared.
Doug