Personally I wouldn't. I feel like it takes so long to "break in" a new T and get to the point where they understand and can actually be helpful that there'd really be no point unless it was a crisis thing. Plus I have a hard enough time telling my T my issues after working together for a year, I absolutely would not want to discuss them with anyone else.

That being said, I can see how someone else might find it helpful, especially as a routine thing. Having to go longer than I'm used to between sessions is pretty hard for me, so I'd understand how having a session with someone else could help.