Originally Posted by
GadgetFreak
I think that they wanted to try two different types of detonators. Also, I seem to recall that the Hiroshima bomb was uranium and the Nagasaki bomb was plutonium. And I think that there was difficulty in making enough enriched material for either. That might have had something to do with it.
I think it was more a matter of what they could make.
Testing the detonator doesn't need a bomb. You can build it and see what happens to pieces of metal without using fissionables.