Complex sentence: Purnell was the first proponent of the idea that two atomic attacks, one following quickly after the other, would be required to end the war, something that Groves and Purnell had discussed a number of times.
Split into simpler sentences: Purnell was the first proponent of the idea that two atomic attacks, one following quickly after the other, would be required to end the war. This was something that Groves and Purnell discussed a number of times.