Someone has said (perhaps many have actually said) that dialogue is the meeting of two phenomenologies. It's what happens when the-way-I-see-it interacts with the-way-you-see-it. But more than just seeing, phenomenology to me involves the construction of meaning out of experience. (Recently, Iris Fodor has written on this, and I believe her article is in the British Gestalt Journal.) This seems to me to be the place where Gestalt appears very postmodern, because one's phenomenology is the truth for him or her but it might not be "the truth" for anybody else (in fact cannot be). No two people experience life in exactly the same way; true believers may affirm identical doctrinal statements, statements of theory, or even organizational principles, but their understanding of what those things mean and how they are to be applied/worked out in their lives will always differ in some respect. The dialogue is the place between where these two experiences of life can meet, but the experience itself is the phenomenology.
In Gestalt we pay attention to what is happening, the unfolding experience, the phenomenology. One therapeutic task is to foster awareness of that, and to be honest about how the total experience is actually a function of the meeting between the two experiences sitting down with one another during the scheduled treatment hour (ie, the client and the therapist). That's why in dialogue we say that the therapist takes responsibility to be present, inclusive, and committed to the process.
Dialogue can be I-Thou, in the sense that two persons actually risk contact with one another, or it can be I-It, in which there is a false self, a rigid caricature of the real life, that is extended toward another, or encountered when reaching out one's self. In this respect a therapist who merely assumes a professional posture, playing a role, doing "therapeutic interventions", but not actually being present with him or her true self, is engaged in an I-It relationship. I-Thou dialogue realizes that there are two, and each is unique; therefore, there will be difference. This difference is the source of excitement, because there is risk in contacting, in being known, and in expressing interest in knowing another.
I am not very steeped in Sartre, and I would appreciate your further description of the categories you, Ed, have shared here, the "In-itself" and the "For-itself." These seem to me to be ways of conceptualizing just one person and so not really a complete way to summarize dialogue, but I may have misunderstood you.