For my simple understanding, this talks about a true/real friend that whenever the situation is they are always being there to help each other. We all know that even though we say that we have a good, real or true friend/s we can't say that we are already perfect as best friend, sometimes there will be a time that we can sulk with each other and also fight or we have controversy, and in that case, we can still forgive, understand and still be best of friend, that's the time that we can say we are best friends. And also, in the last line of the paragraph "two voices, one song" this means about the two best of friends going together with one decision in their way. And that's really the best friend are. Sharing together happiness/joy/ laughter, tears, problems or difficulties in life, and others. Share together and you will definitely be together forever... :)