Photo from the Sukkot Exhibit, curtesy of Neot Kedumim Park
(ב) 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:
(2) With regard to one who establishes his sukka beneath a tree, it is as though he established it inside the house and it is unfit. If one established a sukka atop another sukka, the upper sukka is fit and the lower sukka is unfit. Rabbi Yehuda says: If there are no residents in the upper sukka, the lower sukka is fit.