I’ve read of this sort of thing happening, the people involved were quite well-off. The therapists and the patients were both well-aware of the limitations of the treatment plans and would get together outside of of sessions and trade ideas. Not surprisingly, a lot of people found this improper. If you want to read about it, the book is The Truth: An Uncomfortable Book About Relationships - Kindle edition by Neil Strauss.
One of my best friends is a psychotherapist, and the one that I see is a friend of hers. My therapist once floated the idea of contacting my friend and getting her perspective. I said to knock yourself out. She said she wasn’t sure about the professional ethics and would think about it. She never followed through on it.