I've also never had a duo session in this hobby, but have thought about it on many occassions ... probably will get one by the end of this year as a "special treat" for myself. But, I would never get a duo with girl's whom I've never had sessions with before. It's much better when there is an MPA you really click with, and she suggests her choice of MPA who would make a good duo with her. Even with that, I often wonder if the girls will be comfortable with each other on how they approach the duo session ... when you are one-on-one with a girl, she may do some things (of course, all within MP limits), but when there is another MPA present, she may hesitate to do something.
I know Allure offers duo sessions, and I know that the following girls have done duo's together:
1.) Claudia & April
2.) Kyla & Raquel
3.) Robin & Britney (oh my !!!)