I have a very movable and flexible approach to marriage counseling. My "client" is the relationship and I vary whether I work with each individual separately or both together, depending on what I believe will be most helpful in getting the couple unstuck and able to connect or reconnect. Understood through the lens of ego state therapy (i.e. we all have various "parts to our personality", ego states dominated by particular agenda, emotions, and ways of being ) it's often necessary to help each person individually to recognize and take responsibility for his or her own ways of being before we can make much headway in managing the ways the two people set each other off.
This may look like a joint session one week, individual sessions with a wife or husband for several weeks in a row before another joint session, or meeting with each spouse separately during the same week for a while. Working this way requires granting me permission to use the understanding I reach of each partner's dynamics to help the other relate with them more effectively--and requires a commitment by each partner to not use information as ammunition.
It is an unusual, challenging--and often fruitful--way to work. If one spouse has a mental health condition for which treatment is considered "medically necessary," some insurance policies will cover these relational sessions, but that is becoming increasingly uncommon.