Cannot speak since childhood, writes a book

Hi, the movie I am looking for is about two children that have a severe speech impediment and live in an institute. Nobody can understand them, but they do understand each other. One of them learns to speak when growing up, and he is the only one that can still understand the other person. Dokters test and can confirm that the gibberish that the one person is speaking is indeed a valid communication. They end up writing a book or writing down their story.
I must have seen this movie on TV in the 80′ or 70′ or so. a long time ago in any case.