I did append "Supposedly" because I couldn't find the original source, and I knew the source had retracted some parts of the "story", but I remembered it being about adding tritium to Little Boy as insurance.
The amazing fact about Little Boy is that he was never tested. His development was basically an insurance policy in case the more difficult but more elegant implosion design could not be made to work in time. The scientists were so sure he would work that they didn't think a test was needed. The implosion design was tested at Alamogordo on 16 July 1945. Little Boy was a one-of-a-kind, at least among first-world nuclear powers. All subsequent weapons are based on the implosion design.
As far as the dual survivor question, this obit on Mr Yamaguchi asserts that there were actually 165 people who survived both attacks, Japanese government official recognition notwithstanding.