My H has attended lots of sessions with T1: we started doing couple therapy with him and then just I continued seeing him. T1 is a family systems guy, so he likes to have family members come. H still comes sometimes to sessions for various reasons: support, to talk about a conflict, to learn, etc. H has gone to T3 with me once, she was curious about how he saw my DID. And he went to my last session with ex-T, mostly so that I wouldn't have to drive home (I was not happy about terminating, ex-T was retiring).
I think having a spouse attend a session or sessions can be helpful for some people and with some therapists. I'd say give it a try.