Not saying you won't find what you're looking for, but many Providers have posted that they don't like to be part of these kinds of surprises without knowing that it is something the GF (or alleged willing GF) actually wants and agrees to. And not just your say-so. Sometimes, they also want to talk or email the GF to make sure that there will be good chemistry and no mutual x 3 disappointments.  
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I suppose if you are setting up an "accidental" meeting over cocktails, that would give the Provider (and your GF) an opportunity to get a feeling about how things might go. If your GF pulls you aside and unknowingly says, "Who is that creep? Get rid of her so we can go to the room!" that would be bad for your plan. If the Provider pulls you aside and says, "I don't have a good feeling about this. I think I'd better leave." that would be bad (for your plan).  
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I'm sure you'll get some takers, but maybe some Providers can suggest some things to ask about or watch out for. I am sure you know your GF's preferences and practices. So maybe you can discuss that with a willing Provider (discuss sex ahead of time?) to make sure there's a match. I mean, being ignorantly crude, if your wife likes to munch but not be munched and the Provider likes to munch but not be munched, that would be a disappointing mismatch. If your wife is aggressive and wild but the Provider prefers slow and sensual, that would be a mismatch. Sorry that I don't know to put it better. ... I know what I look for for ME  I just don't know what the ladies look for to insure a good time will be had by all.
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Happy hunting!

Posted By: docdealer69

May add that I might request that this meeting in NYC be set up by me with the other lady as a surprise and leave out the provider part, meaning we would meet "by accident" in our hotel bar, strike up some conversation over a cocktail or two and then move the fun to our room. Not wanting her to know this is all arranged in advance

Ask [email protected]. She can introduce you to the ideal provider who love surprise doubles and more!

Catch me between 12/15 and 12/19

Thanks for the honest suggestions and advice - I agree its an imperfect plan and so appreciate the discussion of pros and cons.  

I usually have chemistry with any female

Big, small, skinny and tall... I haven't met one woman who didn't want to tear my clothes off and munch on my pretty box. I am usually the aggressive one in bed with another woman, but I can switch it up if she wants. I can even teach her to squirt if she's inclined. I am a good actress, but yes like someone else mentioned, I would definitely need a deposit. That way if she's not cool with it, I can be excused and no one is the wiser. Good luck on your search!

I'll be visiting from Las Vegas and would love to entertain you both, a being from out of town is a perfect cover story!  

I specialize in couples! www.curvycoedcarly.com/couples.html

Have you and her talked about a trio before? If so, then a surprise is acceptable in my book. If not, then it might just be your imagination that she wants to do this.

Most/all of the concerns that others have raised are valid.

If you've discussed this a trio of some sort before this, then I'm down to play. I'm brunette, tall, and have some cute curves.

I agree.  This is a surprise situation I would not take part in.    A 3 some is not something you spring on anyone