The counselors in CCS are trained in the approach which believes that marriage/family is a private affair and no one (not even a counselor) should enter into it and so the role of the counselor is to help the couple to own the relationship and make it better.
100% confidentiality is the basic principle of counseling and by having joint sessions, it is highly compromised. Because both of them/many people are present, they do not want to talk openly about their deeper thoughts, deeper feelings and behaviours. Most of the time they end up telling socially desirable things in joint sessions which would not serve the purpose of having counseling.
In joint sessions, many clients when they try to communicate about their perceptions of the relationship, it is taken as a bad mouthing by the other person present in the session and so the other person undergoes trauma while listening to it. So joint sessions may be more harmful, reduce the well being of the clients and worsen the situation which is against the aim of coming for marital/relationship counseling. The counselor would also be seen to be taking sides when he/she tries to empathize with one person which would further hurt the other.
The counselors in CCS follow Teleology (a school of thought in Ethics) which says “what is right and wrong” is based on the consequences and contexts. There is no absolute standard of what is right and wrong. The Counselor will help you to have a non judgmental view about your partner/family member and thereby increase the harmony of your relationship.