The second case of R' Yirmiyah on 9b is when the bottom and the top sukkahs are both passul. I am not clear which is the reason why the bottom sukkah is passul. Is the bottom sukkah passul because it is a sukkah beneath another sukkah? Or is it passul because the skach of the top sukkah is above 20 amos which means that it is passul skach which will combine with the kosher skach below? If this is the reason, why does R' Yirmiyah present the case that the top sukkah has more shade than sun? It seems it would also be skach passul if it is even more sun than shade since it is above 20 amos?

Avraham Sacks, Ramat Beit Shemesh