A person must not answer Amen to his own Beracha (blessing). However if he says two or more blessings then he answers Amen after them. An example of this is the multiple Berachoth (blessings) that we recite after the Haftarah on Shabbath to which the one reciting the blessings also says Amen.
If somebody hears another Jewish person saying any Beracha, even if he did not hear it in its entirety, from beginning to end, he must answer Amen. The person who answers Amen must also have in mind that he is answering Amen on behalf of the person reciting the Beracha. For this reason, he must answer loudly, so that the person who is reciting the blessing can hear the Amen.
For this to work, the person who is saying the Beracha must also have in mind that the Amen is covering him too. The person who answers Amen has to be particular not to answer Amen louder than the one who is making the Beracha.
(See Ben Ish Hai, Yr 1, Parashath Mass'ei)
