+1 on a swing club
Definitely find out the type of lady she might like and someone that will not be intimidating to her. Example: if she's late 30s-early 40s, finding a cute bubbly 20 something may blow up in your face. If your fiance is a little heavy, finding a spinner may blow up in your face.....
Also, it is a very good idea to find out what her expectations are in a FMF. Will you be able to play with both ladies, are you allowed to kiss the other lady, and so on.
I played with a married couple, but I was only allowed to play with her the first few times. He couldn't touch me. Around the 3rd visit, I finally got to give him head, but still he couldn't touch or kiss me.
This is her game, and my advice to you is follow her rules or it may blow up in your face.
Edit: if you go with a provider on this, make sure she is respectful of the rules you as a couple have set. She's there to please both of you and to fulfill a fantasy.