If they had would it have been effective in forcing a surrender from Japan, or would we have had to drop the second one anyway? If it wasn't considered, why not? Was it a matter of not showing our hand without direct consequence?
There were, at levels lower than the President, some discussions about whether a "demonstration" might be a good idea. It was never taken very seriously. I have written about this at some length here. See also this account of the discussion by one of the scientists involved (who changed his story of it later).