My T has also been my marriage counselor and he also sees my daughter. He is (or was originally -- he's had training in various things) a family systems therapist, and I think he finds it helpful to see the family dynamics. It has worked very well for us.

At the same time, I'm not sure all T's could do this. Our T doesn't disclose a lot in general, so it's easier for him to keep confidentiality. It does require extra work and care from the T though. For instance, I was talking to my T about a friend during my own session, and when the friend's name came up in couples therapy the next day my T asked who this friend was, as if he'd never heard of him, so as not to divulge to my husband that I'd been talking about that friend.

I've had bigger problems because of others I know, not my family, who also see him. I got very angry at someone who I knew was a former or current client of his and was reluctant to talk about it, yet I was very upset. He explained that it doesn't matter what he knows or thinks about this other person, it's my session and he's focused on my thoughts and feelings, not his own.

My T does have a lot of practice compartmentalizing in this way, because just about all his business these days is by referral from other clients. I would think about whether a T is very professional and consistent with boundaries, and ask them if they have experience with seeing more than one person in a family, or seeing one person both alone and in couples/family therapy. If they are professional and have done this before, then I think it may be a bigger risk to end up with a crappy MC than to use a T whom is already known to be a good one for both roles. But it's obviously a very individual thing!