Gemara

(Mishnah): If five groups spent Shabbos in a hall (paltry Mechitzos separate them; each has its own opening from the hall to the Chatzer):

Beis Shamai say, each group must be Me'arev (give towards the Eruv in the Chatzer);

Beis Hillel say, it suffices for one of them to be Me'arev.

72b (Beraisa - R. Yehudah ha'Savar): All agree that if the walls reach the ceiling, each must be Me'arev. They argue about walls that do not reach the ceiling.

92a (Mishnah): If a wide Chatzer was breached to a narrow Chatzer, the wide Chatzer is permitted, and the narrow Chatzer is forbidden, for it is like an opening of the wide one.

Sotah 38b: Surely, a short person in back of a tall person receives Birkas Kohanim.

The Aron is not a barrier that inhibits those behind it from receiving the Berachah.

Question: Does [another] barrier block Birkas Kohanim?

Answer: No. R. Yehoshua ben Levi taught that even an iron wall does not intervene between Yisrael and Hash-m.

Rishonim

Question: Our custom is that the Shali'ach Tzibur stands in a Tevah 10 [Tefachim] tall and four [Tefachim] wide with high walls. He is Motzi the Tzibur in Tefilah, and they answer Kedushah and Devarim shebi'Kedushah. What do we rely on? R. Yehoshua ben Levi taught that an iron wall does not intervene between Yisrael and Hash-m, but the Ra'avad explains that this is only when there are nine [others] in the place where the Shali'ach Tzibur is!

Answer (Rashba Teshuvah 1:96): The law of a narrow Chatzer totally breached to a wide Chatzer applies only to separate houses, for each has its own use. Even so, if it is totally breached, it is as if the narrow Chatzer is an opening of the wide one, and people of the big Chatzer walk through the small one. If it was not totally breached, each stands by itself. However, the Tevah is tall, wide and has walls. It is not made for its own use, rather, to serve the Beis ha'Keneses, and people of the Beis ha'Keneses walk through the Tevah. It is like a Tevah in which we place a Sefer Torah. Even if it is very tall, it is not a Hefsek. A tall person, or the Aron, does not block Birkas Kohanim. This discusses even an Aron taller than a person, for it was taught with a tall person in front of a short person. This applies even to a large stationary Aron, which blocks Tum'ah, like it in the episode (Megilah 26b). A Stam Tevah is not a Hefsek, for it is used for the Beis ha'Keneses. The Tevah in which the Shali'ach Tzibur stands is made tall, so all will hear him. Therefore, it is as if he is amidst the Tzibur, so one may answer to him even from the Ezras Nashim. In the Beis ha'Keneses of Alexandria, there was a big Bimah, and the Gabai stood on it and waved flags, so people would know to answer Amen (Sukah 51b). Presumably, also the Shali'ach Tzibur was on the Bimah, so people nearby could hear. The flags were for people far away, like we do nowadays. Perhaps as long as they see each other, it is as if they are in one house, and they join. We find that two groups join for a Zimun for Birkas ha'Mazon if they see each other (Berachos 50b). The same applies if they are in one building.

Poskim

Shulchan Aruch (OC 55:19): If a Shali'ach Tzibur is in a Tevah and nine are in the Beis ha'Keneses, they join.

Gra (DH Shali'ach and DH Af): The Rashba brought a proof from Sukah 51b. Presumably, the Shali'ach Tzibur stood on the Bimah.

Gra (DH Shali'ach and DH Af): This is unlike [a Chatzer breached to another in] Eruvin, for there they have different uses. Here, it is all one use. The Rashba proved from Sotah that the Tevah is not a Hefsek, even though presumably it is big. Also, since they see each other, they join, like the Yerushalmi says about Birkas ha'Mazon, like we said above (Sa'if 14). One can reject all these proofs.

Damesek Eliezer (31): There is no proof from Eruvin [72b], for there the Mechitzos divide. Here, even though it is 10 tall, it does not divide. The Beis ha'Keneses uses it, so it is not a Hefsek. In Sotah, we say that surely the Tevah is not a Hefsek between Kohanim and the people at the time of Birkas Kohanim. There we do not discuss joining, [rather, interrupting the Berachah]. Even an iron wall does not divide! Seeing helps for Birkas ha'Mazon. There, they see each other evenly. Here, they see the Shali'ach Tzibur above. The Tevah is above. It is its own Reshus. Perhaps they do not join, just like Reshus ha'Rabim interrupts regarding joining for Birkas ha'Mazon, even though they see each other! Also, Tefilah is unlike Birkas ha'Mazon.

Mishnah Berurah (59): Also someone else on the Tevah joins. The Shulchan Aruch teaches about a Shali'ach Tzibur, lest one think that we are more stringent about him, since he must be Motzi people.

Bi'ur Halachah (DH Shali'ach): The Gra said that we can reject all the proofs. It seems that even according to the Gra, one should be stringent only about joining to 10. One may be lenient about Tefilah b'Tzibur for the Gabai who stands there, for in any case the Radvaz is lenient even for an inner room, like I brought in Mishnah Berurah 58.

Kaf ha'Chayim (89): The same applies if nine are on the Tevah and one is below (Levush).

Kaf ha'Chayim (90): The Pri Megadim says that this is when they do not see each other. If they see, they join even in another Reshus.

Shulchan Aruch (ibid.): This is even if it is 10 tall and four wide and it has walls 10 Tefachim tall, for it is Batel to the Beis ha'Keneses.

Beis Yosef (DH Kasav): Some question whether a Shali'ach Tzibur on the Tevah joins with the Tzibur towards 10. Perhaps the Tevah is like an Aliyah (second story), and they do not join. I say that it is Batel to the Beis ha'Keneses. It is used for the Beis ha'Keneses. A tall mound or earth in the Beis ha'Keneses joins. The walls of the Tevah are merely for Noy (decoration), and not to separate. This is the custom. This is from Mahari Avuhav. The Rashba says similarly

Taz (5) and Magen Avraham (14): In YD 242 (brought below), we say that the Bimah is considered a different Reshus regarding standing in front of a Sefer Torah.

Magen Avraham: Here is different. Regarding joining it is Batel to the Beis ha'Keneses, since it serves the Beis ha'Keneses. Also, the people see each other. See Siman 195:1 [regarding joining for a Zimun].

Shulchan Aruch (ibid.): Some say that this is only if the walls do not reach to the ceiling.

Beis Yosef (ibid.): Semak says that if five groups spent Shabbos in a hall, and walls up to the ceiling divide them, each one must be Me'arev by itself. If the walls do not reach to the ceiling, one Eruv suffices. This is a support that Mechitzos around the Bimah are not a Hefsek, and likewise for the Mechitzos to the right and left of the Tevah, even though they are 10 tall, since they do not reach to the ceiling.

Bi'ur Halachah (DH d'Hani): This opinion holds that it is not Batel to the Beis ha'Keneses. It is like two houses, one inside the other. This is worse than a Shali'ach Tzibur in a small [Chatzer] when the Tzibur is in the big (Sa'if 17). There, they join because it is totally breached to the big one, unlike here. They hold similarly if any individual is on the Tevah, since there are not 10 without him. It is worse if the Shali'ach Tzibur is there, for even if there are 10 below without him, he cannot be Motzi them in Kaddish and Kedushah, since there are not 10 where he is.

Kaf ha'Chayim (91): Since the Mechitzah does not reach the ceiling, even if was made to separate, they join.

Rema (YD 242:18): Even when the Sefer Torah is on the Bimah, the Tzibur need not stand, for the Sefer is in a separate Reshus.