...with 3-somes has been that the main provider that you are seeing is the one to put together the details of the encounter. You ask her about the possibility, even if it's not mentioned in her website or reviews, and she will tell you whether it's something she's interested in doing. She will let you know if she's had any experience in this area and whether or not she has a special friend with whom she can provide the encounter.

Then you might talk to the other provider, see her website or reviews, and get a sense of who she is -- or you might just trust the main provider and go with it.

If she's not into it, she will let you know and you can proceed with another provider to fulfill that wish list item.

to suggest.  I agree that chemistry-wise it is always better for the lady to choose her partner.  I have however agreed to see ladies randomly on occssion.  When I choose, I am less inhibited with the lady as we have shared our std checks etc.  If you bring someone new, better that everyone gets checked and brings their papers to the session.  One can never be 100% sure but it does give a relaxing place to start from.  

As for scheduling, this is the number one reason multiple sessions of any kind get muddy.  You need to be very accurate in your available times- best to offer more than one date and a range of time frames.  Then stick to the time arranged- no changes etc.  It gets super tough if you begin the tweaking of time and dates especially at the last minute.  Be ready to have a solo session if your schedule gets weird and just has to change PLUS consider tipping both ladies for being flexible to your needs.  Some ladies will want a deposit of 50-100% prior to session as it is not an easy thing to get all arranged.  I have spent hours doing it.  I always find it easier if the ladies are traveling together...still two and then three schedules to merge.  

From a providers prospective I just had a similar experience. I just did my first threesome (FMF) and the other provider reached out to me in regards to her client. We were very comfortable with each other and had chemistry which definitely built on the experience and with me being a new provider she set everything up(from the dinner date to the rendezvous) and kept me and the client in communication which made things stress free for me and him. Ask her about some of her friends in the hobby, what you like/looking for and I'm sure you'll have a great time. Communication is key and it helped make the experience great for all three of us. Best advice would be after discussing things, relax and be ready for a good time.