Is it better to do a threesome with a provider you have seen before and one of her friends

Better than taking a chance on two new providers that you have not met before

Can only get better if she brings a friend that she has already doubled with.
They know what one another likes , they get along well or they wouldn't be offering doubles together.  
Chemistry is great , if you have the opportunity to get a lady you have met and any of her friends, do it.

Never, repeat NEVER, schedule a threesome with 2 gals independently.  ALWAYS make absolutely positive they are aware of each other's presence and that they are BOTH ok with it!    ALWAYS ALWAYS ALWAYS

It is best if they are friends and enjoy working together. like April or Kiwi and I. If they play well together you can't go wrong, even if you have never met the ladies yourself.

have two ladies that have never provided together? Why take the chance that they do not like each other or have no chemistry together? Stick with providers who have provided together for the maximum fun.

Def better if the girls are friends ad have played together before.

Sometimes the unknown can be fun for all ! Just saying !

You can bring together two ladies who have wanted to play together, but have never had the chance.  I remember some introductions that have resulted in continuing 3-sums.