The achronim challenge the chiddush of R’ Akiva Eiger (see previous post and the very good comments there) from two sources that seem to indicate that a single bracha cannot be split between dibbur on one half and shome’a k’oneh on the other: 1) The Mishna (Brachos 34a) tells us that if a shaliach tzibur must be removed, the person who resumes in his stead starts from the beginning of whatever bracha the shat”z was in the middle of. According to R’ Akiva Eiger, why would this substitute shat”z not be able to start from the middle of the bracha, relying on his having heard the beginning (shome’a k’oneh) already? It seems that the bracha must either be wholly recited or listened to, but cannot be split. 2) Tosfos (46a d”h u’lma’n) writes that when people join together for a zimun (which in the days of Chazal meant the birkas hamazon was recited aloud and everyone listened) each bracha may be recited by a different person, but you can’t split one bracha between two people. According to R’ Akiva Eiger, why not? – half the bracha will be fulfilled through shome’a k’oneh, half through dibbur.
R’ Soloveitchik answered these questions using the conceptual analysis of zimun discussed last week. Zimun does not mean each individual has a chiyuv bracha which the leader exempts him/her from using shome’a k’oneh – rather, zimun is a communal obligation, meaning the entire group shares the obligation of reciting one bracha collectively. Where each individual has a chiyuv to recite a bracha, that obligation can be fulfilled using shome’a k’oneh on all or part of the bracha (like R' Akiva Eiger suggested, and which seems implicit in Sukkah 38, as noted in the comments to last post). However, where the bracha must be recited as a collective obligation on the tzibur, though shome’a k’oneh can exempt the chovas hagavra of individuals, it cannot create a cheftza shel bracha that suffices to fulfill the communal obligation of zimun. R’ Soloveitchik explained that the same was true of tefillah b’tzibur, or has he referred to chazaras hashat”z, “tefilas hatzibur”, i.e. it is a communal prayer, not an obligation on each individual to repeat shmoneh esrei that is effected through shome’a k’oneh.