It's worth doing at least once. You'll always regret the wondering if you never try it out. There's something quite unique about one lady sitting on your face while another is riding your cock.
I've had a few and they've been great, definitely a must do. But agree that you need to find girls who are really into each other which can be hard. Them just putting on a show is not the same and you can tell the difference. If needed ask the girl you know if she is into any of the other girls at the agency...i.e if she is at an agency. I am currently trying to find the right combo as have not had one for a while.
I like to treat myself with bi doubles on special occasions and it’s 100% worth it - already thinking about my Christmas treat.
If your regular is offering bi doubles with her friend most likely they get along well and have good chemistry so that’s something to think about. Last couple of times I had bi doubles with my regular I asked her to choose the other lady and she did not disappoint.
A couple of pieces of advice based on previous experiences:
1- Research what you want to do and how you would like the session to go, really important imo walking in having an idea of positions and what you would like to do and get out of it - even though my mind goes blank as soon as they start playing with each other.
2- Book longer than 1 hour if you can - it’s a high investment and time goes even faster with two naked girls in the room plus lots more to do and explore.
I had one yesterday and have had many over the years, helps if the girls are really in to each other and also if you have met both of them, book 2 hours, highly recommended