This is the first part of the paper raising the question in what way two therapists cocreate capaciousness with couples and families. Capaciousness allows helpful perspectives and points of view to appear, which were not available before or in different circumstances. The article is an introduction to further deliberation on the question if co-therapy can be useful both for the therapists’ work and the patients and in what way co-therapy helps the patients to make a change. The paper contains a description of some selected systemic and psychoanalytic conceptions employed during couple or family psychotherapy. Both the systemic and psychoanalytic theories emphasize the significance of the difference – a concept introduced by Gregory Bateson. This article mentions this issue and the next paper will describe it widely. The collected material is the result of individual searches during a long-term psychotherapy practice based mainly on the Milan systemic therapy and taking into account the unconscious dynamics running in families and couple relationships. The article contains practical exemplifications of theoretical conceptions, based on own experiences of working in co-therapy.