Shulchan Arukh, Orach Chayim 57/שולחן ערוך·אורח חיים נ״ז

1Laws of Bar'khu and its responses comprising 2 sections: The prayer leader says: "Bar'khu et Adonai ha-mevorach", and they [the congregation] answer after him: "Barukh Adonai ha-mevorach l'olam va'ed", then the prayer leader returns and says: "Barukh Adonai ha-mevorach l'olam va'ed". Rem"a: And some have the custom that the prayer leader lengthens "Bar'khu", and the congregation say "Yitbarakh v'yishtabach..." while he is extending "Bar'khu" (Tur).

2A place where the practice is to shout at/rebuke their fellows between Kaddish/ Bar'khu and Yotzer Ohr or to speak regarding the needs of the public... they are mistaken. (See above, at the end of Siman 54)