Years ago I saw a back up T for a one-off session while my regular T was away. My T was away for quite some time, so I think I had a couple sessions scheduled with the back up in case of crisis. I was in the throes of anorexia then and wasn't doing well at all.
But for me seeing the T was a waste of time. I had no connection with them and they had no understanding of me at all, even though my regular T had shared some information with them (no idea how much). I didn't return for the other sessions but used different coping strategies instead.
I personally wouldn't do it again with a T with whom I didn't already have a rapport.