I did that once, no twice.
Once, both the therapist and I spoke to the third party (separately) and that person sent a letter assessing the situation to my therapist.
Another time, I phoned the psychiatrist who prescribed for me at the time and asked if he would be willing to let me run a problem I was having with a situation in therapy and he agreed.
There are therapists who do this kind of thing. I would consider using someone I had worked with if I thought they would be helpful.