They dropped the 2nd bomb because:

1. they had it

2. it was a bit different in design from the first one

3. they wanted to see what would happen when it went off, and how destructive it would be

4. they had to justify the prior expenditure and development costs

If they had had a 3rd one, they would probably have dropped it as well, given a few more days in which to do the deed.

Mass murder.

- LH