I think there are many different perspectives on the question and the answer.
What is recovery, is it to close, impact on the 2nd race vs expecations, relative effort and speed at each race and many other race day factors.
I responded with my own perspective and experience.
Something I should add is that in 2011 (betwwen 2010 & 2012 when I did back to backs) I DNFd the 2nd Ironman 8 weeks after the preceeding event.
Lack of fitness played a part in 2012, less margin for error when things didnt go well on the 2nd race. It wasnt entirely fitness though, I had nothing in my legs from the start of the bike, this could have been underlying fatigue that had somehow carried from 8 weeks previously?