Question: Occasionally, I daven in a shul that has multiple minyanim going on at the same time and I can hear another minyan clearly while I’m davening. Is it correct to respond to their kaddish and kedusha, etc?
Answer: R’ Chaim Kanievsky (quoted in Ishei Yisrael 24:n62) holds that one who is davening and hears kedusha from another minyan needs to respond (See Mishna Berura 124:3). R’ Eliezer Waldenberg (Tzitz Eliezer 3:11:4) writes that according to one opinion, even the chazzan would have to respond to the other minyan.
Nonetheless, R’ Waldenberg writes that when part of one minyan, one wouldn’t need to respond to any other. One should try one’s utmost to daven in a minyan where one can’t hear other minyanim simultaneously.
R’ Ephraim Greenblatt (Rivevos Ephraim 1:89:2) writes that according to R’ Yosef Shalom Elyashiv and others, one would only respond to keduasha in one’s own minyan, especially if doing so would disrupt one’s concentration. This would even apply to one saying pesukei dezimra (See Shearim Metzuyanim Behalacha 20:1).
In conclusion, R’ Avraham Yeshaya Pfoifer (Ishei Yisrael 24:28) writes that while one davening doesn’t need to respond to kedusha and barechu, etc. that they heard in a different minyan, they may do so if they want to. One should avoid this situation where possible by davening in a place where they’re less likely to be disturbed from outside.