Can one only answer amen during bentching, if one is inbetween the first beracah and the second beracah and one hears someone finish the first berachah.
Answer:
Yes, this is the accepted custom.
Sources:
Shu"t Bztel Hachochma 4:42, see there for a lengthy explanation of the rules of hefsek in all parts of bentching for different dvarim shebkdusha.
