Reference for Sukkah 1:2
הָעוֹשֶׂה סֻכָּתוֹ תַחַת הָאִילָן, כְּאִלּוּ עֲשָׂאָהּ בְּתוֹךְ הַבָּיִת. סֻכָּה עַל גַּבֵּי סֻכָּה, הָעֶלְיוֹנָה כְשֵׁרָה, וְהַתַּחְתּוֹנָה פְּסוּלָה. רַבִּי יְהוּדָה אוֹמֵר, אִם אֵין דִּיּוּרִין בָּעֶלְיוֹנָה, הַתַּחְתּוֹנָה כְּשֵׁרָה:
If one makes his succah under a tree, it is as if he would have made it in his house [and it is pasul]. (If one built) a succah atop a succah, the upper is kasher and the lower pasul, [for it has two schachs, and Scripture invalidates a succah under a succah.] R. Yehudah says: If there is no "dwelling" in the upper, [i.e., if the upper cannot be used as a dwelling, as when the roof of the lower cannot support the mattresses and covers of the upper], the lower is kasher. The first tanna and R. Yehudah do not differ where it cannot support them at all, all agreeing in such an instance that the lower is kasher with the schach of the upper, its own roof not being considered a roof, so that it is not a succah under a succah. And if it were strong enough to support the mattresses and covers of the upper, all agree that it is pasul. Where do they disagree? When it can support them only with difficulty, as when the lower roof sways and buckles from the (weight of) the mattresses and covers of the upper. The first tanna holds that this is called "a succah under a succah," and is pasul; and R. Yehudah holds that since it supports them only with difficulty, it is not considered a roof and it is not called "a succah under a succah." The halachah is not in accordance with R. Yehudah.]