1) DIVIDING THE LARGE ENTRANCE OF A MAVOY INTO TWO ENTRANCES
QUESTION: Shmuel in the name of Levi says that when the opening of a Mavoy is 20 Amos wide (which is too wide for a Lechi or Korah to permit one to carry in the Mavoy), one should place a board that is four Amos long and ten Tefachim high in the middle of the opening (with the four-Amah length running parallel to the walls of the Mavoy) in order to divide the opening into two smaller entrances.
TOSFOS (DH Oseh) asks that by placing the board there one effectively makes the Mavoy into a Mavoy Akum, a bent Mavoy (see Insights to Eruvin 6:2). Consequently, if there are more than ten Amos between the far end of the board and the far end of the Mavoy, it should need a Tzuras ha'Pesach in the bend even according to Shmuel (see 6a)! Why is this Mavoy not treated like a Mavoy Akum?
(a) TOSFOS (DH Oseh) answers that the Gemara is discussing a case in which there are fewer than ten Amos between the end of the board and the end of the Mavoy, and therefore the bend does not need a Tzuras ha'Pesach according to Shmuel. (Tosfos argues with the teachers of Rashi who require a Lechi in the bend according to Shmuel; see Insights to Eruvin 6:1). If there are ten Amos between the board and the end of the Mavoy, however, Tosfos maintains that the bend indeed requires a Tzuras ha'Pesach.
(b) The ROSH and TUR (OC 363) seem to rule like Tosfos and write that if there are fewer than ten Amos between the end of the board and the end of the Mavoy, no Tzuras ha'Pesach is required. However, their ruling is problematic, because they also rule in accordance with Rav, who requires a Tzuras ha'Pesach even in a Mavoy Akum with fewer than ten Amos in its bend! (BEIS YOSEF)
The TAZ (363:26) explains the opinion of the Rosh and Tur as follows. Normally, a Mavoy Akum consists of one long Mavoy with a bend in middle. In the case of the Gemara here, though, the board in the middle serves as a wall that completely divides the two Mavoys along their entire lengths (except for the few Amos at their ends). This case clearly involves two separate Mavoys that have an entranceway leading from one to the other, and it does not involve a single Mavoy Akum. Therefore, no Tzuras ha'Pesach is required in the bend.
(If, however, the distance from the end of the board to the wall at the end of the Mavoy is ten Amos, then that break is too large to be considered a doorway from one Mavoy into the other. It is considered a Mavoy Akum and needs a Tzuras ha'Pesach even according to the Rosh.)
(c) The RITZBA (cited by Tosfos) says that the Mavoy in this case is not considered a Mavoy Akum, because both openings of the Mavoy are next to each other and lead into the same Reshus ha'Rabim. (He understands that the two entrances of the Mavoy Akum that was in Neharde'a (6b) opened into two different Reshuyos ha'Rabim.)
(d) The RITVA explains that the board cannot make the Mavoy into a Mavoy Akum. It is not placed in the Mavoy with intention to divide it into two Mavoys. Rather, it merely serves as a Heker to split the entrance and create two doorways.
HALACHAH: The SHULCHAN ARUCH (OC 363:33) rules in accordance with the opinion of Tosfos (a), and thus a Mavoy with an entrance 20 Amos wide that was split by a board at least four Amos long is considered a Mavoy Akum. Since we rule that a Mavoy Akum needs a Tzuras ha'Pesach in its bend, such a Mavoy must have a Tzuras ha'Pesach.
The REMA cites the opinion of the Rosh (b), who does not require a Tzuras ha'Pesach in the bend. The MISHNAH BERURAH (363:143) says in the name of the Acharonim (MAGEN AVRAHAM and others) that the Halachah follows the first opinion (that of Tosfos (a)).
(However, see the BI'UR HALACHAH there, where the Chafetz Chaim explains that there is a logical basis for following the opinion of the Rosh, who requires a Tzuras ha'Pesach only when there are more than ten Amos between the end of the board and the end of the Mavoy, and perhaps even for following the opinion of the Ritzba (c) and Ritva (d), who never require a Tzuras ha'Pesach for such a Mavoy.)