After the third blessing of boneh yerushalyim in Birkat Hamazon, the gemara says it is praiseworthy to say amen even by the one saying the bracha. If one says the bracha and then says amen, can another person listening to the bracha say amen after the amen of the mevarech or does the amen of the mevarech end the bracha and make the amen of the listeners an amen yesoma? Many times I see the mevarech say amen pretty quickly after the bracha and it is difficult to say a normal amen before the mevarech himself says amen.
It would seem that amen should not be said after the person who is benching has completed his own amen.
This is an interesting question.
Concerning answering amen after the shaliach tzibbur has begun the next blessing, for instance, if he leaves no time between magen avraham and atah gibor, Maharam Chaviv (Kol Gadol, no. 40) writes that once the second blessing has begun, amen can no longer be answered for the first blessing. His reasoning is that amen is the conclusion of the first blessing, and it cannot be said after the second blessing has already commenced.
[He brings proof to this from a source in the Gemara concerning the shul in Alexandria, where flags were raised to indicate when to answer amen. This shows that one person could not simply rely on the amen of the tzibbur to know when to answer amen, for fear that the shaliach tzibbur had already moved on to the next blessing.] The halachah above is quoted by Mishnah Berurah (124:37) from Shaarei Teshuvah, who brings it from Mahari Molcho.
The problem seems to be specifically the commencement of the next blessing. If, however, somebody concludes a regular blessing, and immediately says something unrelated to the blessing, this might not preclude the possibility of answering amen. The person making the blessing has completed his berachah, and amen can be recited during the immediate period even if the person makes a hefsek.
In addition, both Maharam Chaviv and Mahari Molcho mention proof to their rulings from the ruling of Shulchan Aruch (124:11) concerning somebody who prays his silent prayer together with the shaliach tzibbur, and completes his prayer as the congregation is answering amen. Shulchan Aruch rules that if the congregation has already completed their amen, the individual may no longer answer amen, and this is understood by Maharam Chaviv as being because the Shaliach Tzibbur is already able to commence the next blessing. This is all the more true, he writes, when the next blessing has already commenced the next blessing.
Yet, Rabbi Akiva Eiger gives a different explanation for the halachah mention in 124:11, explaining that the personal prayer of the person completing his davening constitutes a hefsek. According to this rationale, there would certainly be no problem in answering amen after the person reciting the blessing–but not the person hearing it–made a hefsek.
Coming to boneh yerushalayim, there is room to compare the amen after boneh yerushalayim to the amen of the congregation that “completes” the blessing, and moves on to the next blessing. Like the shemoneh esrei, birkas hamazon follows an order, and the amen that a person recites (after his own blessing) after boneh yerushalayim completes the first section of benching, and moves on to the next. Applying the logic mentioned by Maharam Chaviv, this would mean that once amen has been recited, the person saying the blessing has already moved on to the next blessing (at least in potential), and amen can no longer be recited.
According to Rabbi Akiva Eiger, however, there would be no problem in reciting amen, for no hefsek was made on the part of the listener, yet the halachah, especially in view of the poskim’s citation of Mahari Molcho, would follow Maharam Chaviv. It would therefore seem that one would not say amen after the person benching had already completed his own amen.
Note that this ruling does not concur with the ruling given in Notrei Amen (p. 90), who qualifies Mishnah Berurah’s ruling to cases in which a person fulfills an obligation by means of hearing a blessing. According to him, the ruling would apply, it seems to a mezuman, but not otherwise. Although his rationale is possible, he apparently did not see the teshuva of Maharam Chaviv, whose reasoning does not appear to match this assessment.
For most blessings, however, it is true that an ordinary interruption on the part of the person making the blessing would not preclude the possibility of answering amen (as explained), and our answer applies specifically to the amen of boneh yerushalayim, after the person benching has already completed to say amen. Those who are benching should be warned not to say their amen too fast (like poskim write of a shaliach tzibbur’s commencing the next berachah too fast).