Japan was already prepping to defend the Japanese Home Islands against an expected invasion by the US military in the next few months. Why would they suddenly reverse course and surrender unconditionally because of the Soviet Union taking less populated and less important northern islands, when they were going to defend Kyushu to the death?My opinion:
The first bomb was justifiable. If the Allies had waited after that until the Soviets invaded, Japan would probably have surrendered, making the second bomb less justifiable.
Had the Soviets invaded without a second bomb, the Japanese may not have surrendered until Soviet troops actually landed on the home islands, which potentially could have led to a partition of Japan similar to Korea. This would most definitively have NOT been a good thing.
Beyond fortifying Kyushu, they were also doing the same with Honshu, turning every possible invasion beach and inland route into a death trap that would dwarf Iwo Jima or Okinawa.