Daf Yomi - Sukkah 18 - Halacha L'Moshe Misinai x 2

The Gemora cites a dispute between Rav Acha and Ravina if we apply the principle of lavud in the middle of a sukkah. If there is an area of open space less than three tefachim running across the entire length of the sukkah, we would need to attach the two sides together to close the gap in order for the sukkah to be valid. They agree that lavud can apply at the side of the sukkah. If there is an open space wider than three tefachim, it must be filled up with s'chach to ensure that the gap is lesss than three tefachim.

The Ran rules that the s'chach must be placed adjacent to the s'chach and not on the side of the wall. If placed by the wall, the sukkah will still be disqualified because we would need to rely on two halacho l'moshe misinai principles, firstly - dofan akumah accomplishing that the s'chach which is next to the wall is deemed to be part of the wall and secondly - lavud will subsuquently close the open gap and attach the s'chach to the new wall. Two halachos cannot be applied at the same time, thus the sukkah will be disqualified.

This is parallel to another of the Ran's rulings regarding more than four tefachim of ineligible s'chach adjacent to the wall and the wall of the sukkah does not reach the s'chach. To validate the sukkah, we must apply two halacho l'moshe misinai principles and that cannot be done. We must extend the walls to reach the s'chach with the principle of gud asik and then apply the concept of dofan akumah for otherwise the ineligible s'chach will disqualify the sukkah.

Reb Akiva Eiger explains that the Ran does not simply mean that two halachos cannot be applied in one instance, rather if one halacho is dependent on another halacho l'moshe misinai, they cannot be applied. Each of the halachos is required to stand on its own merit. In the latter case of the Ran, we cannot apply the principle of dofan akumah until we rectify the wall to remove the air space. (Dofan Akumah cannot be applied when open space is present.) Gud asik must be applied first to extend the walls and then we can contemplate dofan akumah. The Ran maintains that this is not allowed.