1. One who has finished the blessing of “Habocher be’amo Yisrael be’ahavah” and had not yet commenced the recital of the Shema who hears the cantor or any other person ending this blessing, the opinion of the Shulchan Aruch is that one should not respond “Amen”, as it would constitute a break between this blessing and the Shema. (Shulchan Aruch 59)
2. According to the opinion of the Rema one is allowed to respond “Amen”, and it is not considered a break, as there is no direct correspondence between this blessing and the Shema. Instead, the Rema considers them to be separate blessings, which Chazal decreed that we should make before the Shema. Nevertheless, it is correct to finish the blessing together with the cantor, so that one is not obligated to say “Amen”. (Mishne Berurah ibid)
