Here is my experience.
I realize that things went extremely well for me, and that not everyone will have this experience.
At least I think so. I mean I was in the middle of a drug interaction from hell and couldn't even drive.
But....my boyfriend thought it was positive and he pretty much impressed the hell out of my therapist.
So let's see....
My boyfriend has known about my ptsd practically from day 1. I will say that we are only about 3.5 months into this relationship. He has been supportive and knows a LOT about inner child work as he's done it himself. (I am focusing on inner child/parts work right now.) He's read everything I've sent him on PTSD. He tries to help me through episodes and doesn't hesitate to set boundaries. It's also important to note that he is an "N" (bold, in size 98 font), in the Myers Briggs personality type, N standing for intuition. And since he's a guy, that pretty much makes him a freaking unicorn. Lol. I mention this as his intuition is spot on, but I know that most men, heck, even most women, aren't as intuitive as he is, so they probably won't pick up on this stuff as readily. (Yeah, his intuition blows me away----).
So my boyfriend had been doing things to help me and it was good that he was able to get feedback from my therapist----someone who has worked with traumatized kids at Shappard-Pratt----and she told him that what he was doing to help me was on target. My therapist was also able to tell him other ways to help me in the moment. She reiterated my 1-5 anxiety and shut out scales, explaining how when I reach a certain level I have an inability to problem solve so communication needs to end.
There was more-----just sharing the highlights.
Oh, and my boyfriend does NOT have permission to talk to my therapist outside of joint sessions. I'm not ruling this out in the future, but for now, no.
We have also agreed to have joint sessions via phone in the future if needed.
So I guess what I'm trying to say is that joint sessions can indeed have a very positive outcome. I'm willing to share more if you'd like me to. I just don't want to see what can be a very positive thing-----being ruled out without thorough consideration.
Maybe you aren't ready right now, and that's ok, too. It's possible that a joint session isn't what you need right now. But based on your post, I think a joint session really could help your husband "get it".