I generally don't cope very well with my T's vacations for a few different reasons, so usually I have a downward spiral that -- by definition -- occurs exactly when my T is unavailable.

Last spring I did a one-off session during my T's week-long vacation. My T suggested an independently practicing colleague in the same office suite who she thought would be a good fit for me. At the time, regular T and I were at an impasse about a particular issue, so I wanted to see what backup T thought about it. She was slightly helpful with that issue but not really, which was a bit disappointing. I thought she might have a big insight I hadn't considered before.

However, I did think it was useful to have somebody to talk to during that week, and I liked being able to go to the office like usual. It mitigated the disruption in my routine. I actually found this T very helpful for suggesting concrete coping strategies to get through the week, which is not really my regular T's jam. I would see backup T again for a one-off if needed.

So I think you should do it if you're at all intrigued or interested. If the backup T is good, then you might get something different from them. If not, at least you'll appreciate your regular T more.

I do know that regular T and backup T shared info about me before and after the session. I didn't really care much what they talked about because I trust my T implicitly to share only what she deemed relevant, and I didn't share anything with backup T that I didn't want regular T to know. I do admit to being slightly curious about their conversations, though.