Our shul expanded by replacing the wall between the main hall and the beit midrash with a (only partly
soundproof) glass+curtain partition which is opened on Shabbat. There are many minyanim in the two halls and the upstairs ezrat nashim and library morning and evening, almost like shtieblach but at set times.
Many participants in the beit midrash minyan on weekdays answer kedusha, birkat kohanim etc to the minyan in the larger hall, usually but not exclusively when the BM is between or after minyanim, sometimes even opening the partition to hear better.
A person that is in middle of davening does not have to answer to other minyanim, (according to most poskim). If answering kaddish or kedusha is not going to ruin his concentration on what he is davening, ten it is preferable to answer even though he doesn’t have to, but if it is going to hurt his concentration he shouldn’t answer.
Rivivos Efrayim1- 69 (2)in the name of R’ Eliyashiv and R’ B. Zilber zt”l, Tzitz Eliezer 11-3, Leket Hakemach Hachadash 66-25, Eishei Yisroel 24 ftnt. 62.