I had a specific issue I wanted to talk about with another T, but it involved my therapy or my perception thereof and not something just about me. I had one consultation, felt it served what I needed, and then I told T about it afterwards. He appeared to be surprised but he was all for whatever I wanted to do (the opposite of non-controlling) and said I didn't need to run anything by him. I asked specifically if this was a kind of dishonor to our relationship, and he said no. I tend to think this is how T's "should" approach a client who wants to work with another therapist.

Although you know you don't have to include your T in on your plans (nor do I think it is wrong to keep it to oneself), you want to, and I would encourage you to do that at whatever time makes sense. I think that he could be helpful in helping you think about how to work with someone else and/or this issue, because he knows you and because it may impact your therapy with him and him being informed seems to me like that would help. Maybe he's completely irrelevant to the conversation, but I kind of think from your postings here that he might be skilled at guiding you in how to approach this work you want to do. My version of this is that my T has been helpful to me when interacting with physicians and the like.