My story is a bit different. In my case, my husband also started seeing my T several months after I started which completely fine with me and if I remember right it was my idea. So, T learned a great deal about our dynamics just by working with us individually.

At some point (and I can't remember how far into the process it was), T had us both come in for more of a couple's session. It wasn't my session or his session that was used for that meeting; it was a session independently run. I think that would be important to clarify with your therapist if missing out on your individual session would be a problem for you.

We have had several joint sessions over the years. Sometimes they've been helpful; sometimes I felt pretty neutral about it, kind of eh. Before your session, I would ask your T for specifically what she hopes to say to him and specifically what her goals are for the session. Don't go blind into it.