> people who are ready to surrender don't need to be nuked twice.

The question remains whether in fact Japan "needed" to be nuked even once - it's unknown whether the ongoing city by city destruction would have resolved WWII.

Further, did they "need" to be nuked a second time so quickly (within three days)? That second nuke was not to force a surrender (just as the first wasn't) - the two bombings were two live tests of two separate weapon designs untested in the field of war.