Why Two A-Bombs?

Because "the Manhattan Project’s... leaders had to justify the $2 billion ($26 billion in today’s dollars) expense to Congress and the public" — Hiroshima AND Nagasaki: The Inside Story:

"How would you get Congress to appropriate money for atomic energy research [after the war] if you do not show results for the money which has been spent already?" …the U.S. had produced two types of bombs–one using uranium, the other plutonium. Whenever anyone suggested that the moment the bomb was dropped the war would be over, [bureaucrat] Groves countered, "Not until we drop two bombs on Japan." As [historian] Goldberg explains… "One bomb justified Oak Ridge, the second justified Hanford." Hiroshima was hit with the uranium bomb, nicknamed "Little Boy"; the plutonium bomb, "Fat Man," was used against Nagasaki.

A collection of previous posts on the topic — Some Thoughts on the A-Bombings of Hiroshima and Nagasaki.