I had a backup T when T was out of the office for a few months a couple of years ago, and once I saw a T for a "consultation" another time when he was gone. I did find it interesting to see how a new person felt, sort of like test driving a new car. I felt both a little more exposed and also a little more able to see myself clearly, rather than plunking down in the comfort and familiarity of t's office, if that makes sense.

I have no idea what my T might have told the backup T. I think it might be a good idea for you to just allow this to happen and not try to micromanage the information or turn it into a thing where you are worried about what someone else thinks of you. I don't mean to be insensitive and of course you have a right to control the flow of information from your T, I just think it might be a good exercise for you to just focus on getting the support you need, not worrying about what this other T thinks or knows.