I find his response kind of odd. It seems like he shouldn't mind if you liked "his" backup T better and switched to them because he should encourage you to get the best "fit" you can find. (Or more accurately, he can be hurt but deal with it himself, rather than persuading you to feel guilty or not do it.)

The impasse I discussed with backup T actually did have to do with my T and her boundaries (or what I perceived as her infuriating rigidity at the time ). Regular T actually encouraged me to discuss the issue with backup T because she thought maybe an outside perspective would help me get unstuck. I had the sense that my T felt confident about her perspective and how she conducts herself in our relationship, so she wasn't bothered by her trusted colleague weighing in.

I also had a period recently where I was really upset with my T (for vague reasons), and I spent an angry evening looking through Psychology Today for new prospects. After I calmed down, I told her in session that I felt guilty, like I had downloaded Tinder when I was already married. She asked if I had found anybody who looked promising, and I told her that I was half-considering [this guy who rents office space from her who I happen to know is a close friend hers]. Then we joked about me sneaking around with her colleague in her own office suite. Granted, we were joking and I wasn't really thinking about switching, but she has said before that maybe I could consider working with him on stuff with my father. (I think she thought I would develop paternal transference, but I bet she's wrong.)

Anyway, my point is that it seems weird that your T would be uptight about any of this. I hope you are able to clarify his position in the next session, but I also hope that you do whatever feels right to you.