We have already explained before at the beginning of chapter 2 that in the word "basuccos", in the verse "You shall dwell in succahs for seven days" the letter "vav" is missing, thus indicating only one succah; chazal derived from this that there is a prohibition to make one succah under another succah. Likewise we learn from here that there is a prohibition to make a succah under a tree or in a house.2. Does this mean that when one succah is under another they both are invalid?
No; when one succah is under another only the lower one would be prohibited. The top succah would be kosher, provided the s'chach of the lower succah (which is the floor of the upper one) is strong enough to hold the weight of the person sitting in the upper succah together with his blankets and pillows, even if only temporarily (Shulchan Aruch, section one), i.e., even if the roof of the lower succah shakes when the person is using the upper one (M.B., section 3). There should also be a distance of ten tefachim between the s'chach of the lower succah and that of the upper succah (Shulchan Aruch, section 1). Whenever the upper succah is invalid it does not disqualify the lower one.3. What would the halachah be in reference to the lower succah if the upper one has more sunshine than shade?
As was mentioned, in order to prohibit the lower succah the upper succah would have to be completely kosher. In this case, since the upper succah has more sunshine than shade it is not a kosher succah and cannot prohibit the lower one. This would apply even if the lower succah could support the occupant of the upper succah with his bedding as mentioned before (M.B., section 7; Shaar Hatziun, section 9). Therefore, if the lower succah has the requirements of a kosher succah it would be permitted. (See also further on, question 13.)Next Chapter