These questions are debated in the Talmud and the conclusion is “meitev yatvenan, beruchei lo mevarachenan,” which means one should eat one’s meals in the sukkah on Shemini Atzeret without reciting Leishev Basukkah. The Rif explains that reciting Leishev Basukkah, while at the same mentioning the day of Shemini Atzeret in the Kiddush, would be a contradiction in terms. Furthermore, it would be an insult to the Yom Tov of Shemini Atzeret to recite Leishev Basukkah because this might send a message that the day is Hoshanah Rabbah /Chol Hamoed.
Why, then, asks the Ran, are we allowed to recite the blessing for counting the Omer on the 16th day of Nissan, which is the second day of Pesach? Should we not also be concerned that reciting the Omer blessing on the second day of Pesach sends a message that the second day of Pesach is really a weekday? After all, the Torah tells us that the first day of the Omer must be counted on the day that is “mimocharat hashabat,” the day after the festival?
The Ran gives two answers to this question.
First, the Sefirat HaOmer blessing and the Kiddush blessing are done at different times and different places and they celebrate different things. The blessings of Kiddush and Leshev Basukkah on Shemini Atzeret, on the other hand, would be recited one immediately following the other, before eating in the sukkah and they are therefore incompatible.
Second, the mitzvah of Sefirat HaOmer is, according to the Rif, a Torah requirement whereas the requirement to honor both days out of doubt is a rabbinical requirement.
As far as ba’al tosif goes, the authorities explain that there should be no concern, as long as there is some outward manifestation – heker – that one is sitting in the sukkah “out of doubt” and not for the purpose of fulfilling the mitzvah of sukkah. Such a heker is achieved by omitting the blessing of Leshev Basukkah, because any bystander observing this conduct would understand that one is sitting in the sukkah merely to enjoy the breeze on a hot day and not in order to fulfill the mitzvah of sukkah. Sleeping in the sukkah on Shemini Atzeret is prohibited according to some halachic authorities because one does not recite a berachah for this and so omitting the blessing would not constitute a heker.
Other halachic authorities suggest other ways to broadcast the fact that one is sitting in the sukkah out of doubt and not in order to fulfill the mitzvah of sukkah. For example, the Tur suggests not eating dinner in the sukkah on the evening of Shemini Atzeret. Others maintain that as long as one is not sitting in the sukkah with the intention of fulfilling the mitzvah of sukkah but only “out of doubt,” there is no issue of ba’al tosif. However, all agree that in order to avoid ma’arit ayin – the outward appearance of ba’al tosif – there should be some heker.
Why, asks Tosafot, do we not fulfill the mitzvah of arba minim on Shemini Atzeret out of doubt? Tosafot answers that arba minim are muktzah on Yom Tov and that unlike sitting in the sukkah, which may be done for non-mitzvah purposes, there would be no other reason to use arba minim except for the mitzvah. It would therefore cause a perception of ba’al tosif.
So what should we do? “Eat both meals in the sukkah on Shemini Atzeret without a berachah,” rules the Shulchan Aruch, who lived in Israel. In colder western climates, the omission of the Leshev Basukkah blessing may not be as clear a heker. Therefore some Ashkenazi Jews have the custom of making Kiddush in the sukkah on the day of Shemini Atzeret and then leaving the sukkah and eating lunch inside the home. In these matters one should follow the custom of one’s tradition.