I assume you're going with your H to old T. When my H and I both saw my T together it wasn't the same because I had to share him. I didn't like that, but I found it productive. I was still seeing her individually, of course, so that's different from you. There's always the chance you'll develop those feelings again, but I think you had a very good closure with your T and put those feelings behind you. I think it's going to work for you.

If the attachment happens again, you will be more prepared for it. Maybe you can tell yourself, "it's just that attachment again, and it's okay. I can deal with it". Or, if it doesn't work out with old T, you can see the other one, can't you? I do think that for a lot of us, the attachment is going to be there no matter what, and we have to accept it, work with it, and not see it as our "enemy".