The second bomb was simply dropped because the US had not yet received an unconditional surrender after dropping the first. This is an interesting discussion thread with a lot of assertions about motives. Truman definitely wanted Stalin to be wary of the new US monopoly of this weapon but Stalin was very shrewd not to show any concern when at the Potsdam Conference Truman disclosed that they had successfully tested the weapon. Stalin probably already knew about it. The seeds of the Cold War were already planted. The future arms race was on before WWII had ended.