135:5 In the evening, after returning from shul, one enters the succah and recites kiddush immediately, but only if it is definitely already night. When he recites the bracha of “leishev basuccah” (that G-d commanded us to dwell in the succah) for kiddush, he should intend for it also to include the meal, sleeping in the succah, and anything else he might do in the succah until he recites kiddush the next day. When he recites the bracha “shehechiyanu,” he should intend for it to apply to both the holiday and to the mitzvah of succah. Therefore, on the first night one recites “leishev basuccah” first and “shehechiyanu” after it so that “shehechiyanu” is also on the mitzvah of succah. (If one said the brachos in the wrong order, he does not repeat them – Mishnah Brurah 643:3.) On the second night, one recites “shehechiyanu” first and “leishev basuccah” after it.
135:6 When several heads of families eat in the same succah together with their wives and family members who need to hear kiddush in order to be included in it, if all the heads made kiddush simultaneously, it is not valid because the kiddush will not be heard clearly. Therefore, it is better that they should make kiddush one after the other.
If they did make kiddush simultaneously, such as in a case where there was no one else there who needed to hear kiddush from them, and one of them preceded the others by finishing the bracha of hagafen – or one of the other brachos – before them, and then his friend finishes after him, the first should not respond “amen” to the other’s bracha because doing so would constitute an interruption between his own bracha and drinking the wine. The common practice is for each to wait for the others and to respond “amen,” but this is not the law and they should all recite kiddush together in this scenario.