I could be wrong, but the advice everyone gave was letting the lady he was in contact pick the other lady when he stated in his question that the other lady was already selected.
Sorry, but Q's advice was useless to the point of almost being insulting. I suggest Q re-read the actual question asked and respond to that. What you gave was an unsolicited opinion. If that's what we are doing here, I would offer: don't do it. I've had threesomes and a foursome. I don't find they are double the fun. IF money is the concern, have two separate parties for the same money, IMO.
Also, if you are all going to accept the advice to let the lady choose, also accept that she is working for herself and not for you. Obviously there are some combos that work better than others, but at some point the client is the client; the lady is not the client.