Unfamiliar co-therapists can use code phrases to transfer control of the session. Both they and the client can then focus on the client’s issues rather than the dynamics between the therapists.
While there should be no confusion in the mind of the client as to who is leading the session, there may be some stress for the therapists. Trainees may wonder if and when their supervisor will take over (or in some cases, may wish their supervisor to rescue them!). Supervisors may wish to ask a question or reinforce a point, but hesitate to undermine the trainee by interrupting.
The client’s focus should be on the issues they bring to therapy. Any awareness of unease on the part of their therapist may distract from this focus. Transfer of control of the session from one therapist to the other should be obvious to the client, but wrangling between the therapists should not.