Mishnayos Sukkah Perek 1 Mishnah 2

כאילו עשאה בתוך הבית – and it is invalid.

תחתונה פסולה – for it has two [sets of] S’khakh/coverings of the festive booth, and the Biblical verse invalidates a Sukkah that is underneath [another] Sukkah.

אם אין דיורין בעליונה – which is not appropriate for dwelling, such as [if] the lower roof is unable to endure/sustain pillows and cushions of the upper [Sukkah]. But the First Tanna/teacher and Rabbi Yehuda did not disagree if it cannot endure it all, and in that, the entire world admits that the lower [Sukkah] is valid with the S’khakh/coverings of the festive booth of the upper [Sukkah], for its roof is not considered a roof, and it is not a Sukkah that is underneath a Sukkah. But if it is healthy and cable of receiving the pillows and cushions of the upper [Sukkah], the entire world does not argue that it is invalid. But they disagree when it can receive it in an emergency/when needed, such as the case where the roof of the lower [Sukkah] shakes and vibrates from the pillows and cushions of the upper [Sukkah], where the first Tanna holds that in this case it is called a Sukkah that is underneath a Sukkah and is invalid, and Rabbi Yehuda holds that since it other than in the case of an emergency cannot accept the pillows and cushions of the upper [Sukkah], it is not considered a roof, and this is not a Sukkah that is underneath a Sukkah. But the Halakha is not according to Rabbi Yehuda.