I've never experienced a duo ever in my life.  I'm thinking about one, any general advice on setting them up?  What are your thoughts on them?  From providers and clients perspectives?  Are they generally worth it?

Definitely look for providers who have worked together before for good reviews.  Do you want all the attention or do you want to watch them get it on with each other too?
If they work well together you will be immersed in pleasure, if not, disappointment.
Likewise, you only have one cock, so unless you are a multi shot guy in a short time, be prepared for  multiple hours so you can enjoy both ladies.
In my experiences I have had disappointing duos and awesome duos.  The disappointing one had me drunk going in with the 2nd provider just "there".  She was a waste and the other one was a "cum and done" type.  Totally unsatisfying, I wasn't at my best and the girls just didnt click w me or w each other.   The better duos I have had had me eating out one gal while the other rode me or blew me, then they switched.  While I was fucking one in k9 she was eating out the other gal who was staring into my eyes as she came.  In 2 hours you should have either a hand, a mouth, a pussy or an asshole on your cock 80% of the time.  Change positions often, its fun and keeps you from cumming so quick.  Oh, and if you want to slip out of one and into the other, use Female Condoms... seamless transition.
There, thats my advice from my own experiences, do with it as you will.

Any guys I've ever talked too who had a threesome with civvy girls, the women simply let him in on their fun. My experience was different...the two girls were there for me! I was the focus of their attention.

FMF duo in their lifetime.

What's the old saying?  The worst day fishing is better than the best day working?  Kind of fits here.  I've done 2.  Completely different experiences.  
Should I do it again,  I would choose this way
1. Someone that I have seen and really clicked with  (lady #1)
2. She has done duos and preferably has reviews for duos
3. Ask her to give me the names of 2 or 3 ladies she would really like to do a duo with  
4 Look at the list and choose the one that I want the most
5 Let lady #1 set it up

Decide what you're looking for: two ladies to please you or to watch two ladies have their own fun. Contact someone who offers duos and see who she offers them with. Pick one of her friends. Viola! You have a duo

Have a regular set it up with a friend of hers.  Be sure she understands what you want.

Book two ladies for two hours.  I've done it a half-dozen times.  What separates the good ones from the so-so encounters is whether the ladies are bisexual or not.  As a man, I need a certain refractory period after the first pop before being ready for round 2, so what better way than a private show of two ladies getting it on.  (If Lopaw wanted to join in, I would be okay with that, but I'm not paying extra.)   I find it shortens the refractory period by many minutes.  Without the girl-girl action, its basically just two ladies tag-teaming you.   Men are visual creatures and watching two ladies go at it is very hot.  

Try It You'll, Like It. .at least once in life....Your Own little  2  girl Harrem..

The monger will probably be bringing his own.

Always with at least one lady that I've had the pleasure of seeing by herself. I've usually gone with her suggestion of lady for the third.  
Mostly, the ladies have been together as a team for a while and really enjoy each other.  
A couple of times I asked one regular if she would do one with a lady that I suggest and either they have said that they have done one before with that lady or that they have wanted to fuck them and would love the opportunity.  

And yeah, I know. I was terribly redundant. Caveat at the end. Hahaha.

The first and foremost Q about FMF duos is to establish whether you are OK with a tag team session or not.  I have done duos that were tag teams except for the obligatory double BBBJ. Not worth paying double for that.
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Assuming you want more than just a tag team, the next thing is to find a duo that will deliver more of what you are hoping for. Read reviews of duos by searching "and" in the name field.  You will get some singles such as ANDrea, sANDy, cassANDdra and so on, but it's easy to recognize real duos. You can also search "&" in the name field and get a bunch of authentic duos.  (There are usually more "and"s than "&"s.)  
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As others have or will say, it is often better to meet with a duo where you already know one or both ladies. Put your faith in a trusted Provider to help to set things up with a mutually compatible third party. Etc.

...focused on your pleasure? It's fucking unbelievable! One's trying to suck you dry while the other is trying to smother you with her VJ. It's non stop sensory overload until you collapse. The key is to find the right two ladies.  

the "right" two, I'll still take them.   Lol

I personally experienced my first date of three with an agency I like to go through sometimes. It is the only agency I will go through. I have posted a review of that date of three on TER, go take a look at it.  

Invitation to a 'gay for pay Latina' mess.

I have always only booked for one hour at a time for singles.  Why do you say to only go for at least two hours?  I mean yeah more time is great but what makes it more "essential" for a duo?

I guess if all you want is someone to get you off,  2 women is a waste. If you want intensity, two different BBBJs and the different sensations that go with that. Plus two different sensations from coitus and licking pussy at the same time and then switching to doggie with the other woman, while she is DATY of the second, two hours is just amazing.  
In my first couple of 3sums, I tried 90 minutes, and I ended up extending. It just wasn't enough time to savor all that was available.  
Damn, it's so much fun...........  

I booked two hours and finished just under the wire. No way 60 minutes or less is enough time to explore two women

I've had many duo sessions, both with providers and in my civvie life.  It's like pizza; when it's good it's really great. But when it's bad it's still pretty good. :p  

Also make sure the girls play with each other. I had an appointment with a pair that barely touched each other.