I had this hapen one time.  All my doubles, the ladies have been friends and encouraged me to see the other one.  I saw the lady I've never met one night, then had the duo the next.

Hate to break this to anyone but this is P4P. Isn't that a cute way to say it?
It's sex, for money, with whores. (If you don't like that, look up the definition. Don't hate the player, hate the game.)

This isn't eharmony, or the sisterhood of the traveling dildos. The client is not your boyfriend, property, or exclusive penis with a wallet.

If you don't want the "new provider" to see the client, you can pay her the fee she would have made.
How's that?

No one provider owns a client or his money.  If you don't want to risk losing a customer it's best not recommend another lady for any reason...ever!

This poll is mainly for providers but I am curious on everyones input!!

1. If a provider recommends a client to do a double another provider and for some reason it doesn't work out in the scheduling department, if the client contacts the new provider for a solo visit what is the proper ettiquette?

A. See the client
B. Tell the client he has to wait to do a double with his original provider 1st
C. Ignore the clien'ts request as if he doesn't exist

D, a makeup doubles session where all can attend,  now there are no surprises, he knows both of you and has a general idea what he is in for

See me lol

Unless the the client did something wrong beyond scheduling issues, why would the other options you listed apply?

This is the "hobby."  Everyone is a free agent here.  
That's the beauty of it.