1) If someone is making the blessing of “Baruch she’amar”, and while doing so he hears someone else finishing a blessing, there are three different possibilities: 1) If he has not yet said the words “Blessed are you etc.” (the first time), he may answer Amen. 2) If he has started the words “Blessed are you, Hashem… the G-d, the merciful father, etc.”, he may not answer Amen for the blessings of “Baruch she’amar” or “Yishtabach”, but he may answer on other blessings. 3) After saying the second “Blessed are you Hashem”, if he has not yet finished the blessing, he may not interrupt at all. (Shulchan Aruch and Mishne Berurah 51)

1)    2) One may not interrupt the prayer by speaking, from when one says the blessing of “Baruch she’amar” until one has finished the “Shemoneh esrei”. On those days when we say the “Tachanun”, one should not interrupt until after “Tachanun”. (ibid)