It depends on the therapist. There are no hard fast rules about this. My therapist does individual therapy with me, my husband, and my son, and on occasion he has also done marriage therapy with my husband and I. He does a great deal of family therapy, so he is quite comfortable working with members of the same family, and my husband and I have no problem with how it works for our family. We actually have really benefitted from his knowledge of the family dynamics that work between us.

Some therapists won't do it; they consider it a conflict of interests. I suspect they don't have the training in family systems and family therapy. If they are not comfortable with it, it probably isn't going to work for them anyway.

You need to consider that in a marriage therapy role, you T will not be siding with you; he will be working working with both of you and your relationship as it works as a dynamic. Is it going to bother you if your T spends time focusing on your husband in a session, or if you feel he supports you husband in a particular situation? If so, then a separate therapist might be a better idea.