Is there a best way to find the reviews on here that include threesomes (ie. Provider A and Provider B)?

That’s all just “and” under Name. Then fill out all the other criteria you wish to filter by, such as City.

If  you  have  a  specific  Provider  in  a  particular  area
with  whom   you'd  like  to  enjoy  "Doubles",   search
in  said  area,  said  Provider's  TER  Profile  and  select  
"Double  Reviews".  

Half way there. Also search for the & symbol.  
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Some duo reviewers use "&" in the name field. Some use "AND" in the name field. You have to search BOTH for better coverage but you'll get false positives, too.
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"&"  will return all "Name1 & Name2."
"AND" will return all "Name1 and Name2" but will also return ANDrea (solo), amANDa, cassANDra, and so on.  
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Good luck.

So very few reviewers ever use it. I just did a NYC city search using "&" and the most recent hit I got was 4 years ago. So not real valuable but it only takes a few seconds so I guess to doesn't hurt either. lol.

Thank you to everyone who replied. I will make sure I try these!

…check their individual profiles. When a provider de-lists herself or is de-listed by TER, her INDIVIDUAL reviews are removed but her DUO reviews remain listed.  

Another reason to do this is many providers have 1-3 reviews as a duo which isn’t always the best measure. Or they’ can older reviews. Cross checking their individual reviews can give you a better look into their rating.

Great advice although I do have a pet peeve. I wish reviewers would clearly state in the review if the girls "tag team" or "fully interact " with each other.  

I sometimes play with my partner who is a bi f. You just learn over the years and word of mouth who works good with couples. But when we first started doing this , there was certainly a learning curve. Word of mouth through other couples is the best way. So girls who enjoy couples might have a few good couple reviews. Look for them.

I'm making an assumption here, but ...
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If an M-F (especially M-biF) couple seeks an appointment with a solo Provider (P) it is very likely that the M-F are looking both M-P *AND* F-P interaction ... unless the F of the couple just wants to watch a reverse cuckold session.  All three should probably clarify expectations ahead of time.  
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If a solo-M asks about arranging a threesome, the Provider(s) might assume that the M is mostly interesting in the F-M-F action and may or may not be interested in watching F-F action from afar or being the third wheel in some F-F-M action.
All three should probably clarify expectations ahead of time.

I'm aware of one in particular because I met the bi-F part of the couple on this board. She was a defiantly out-of-the-closet whoremongeress who had a boyfriend she called "PIC," for Partner In Crime. They operated as a team, seeing providers in their part of the country and indulging pretty much equally from the sound of it in each other and the provider.
She was perhaps the horniest woman I've ever met and invited me to visit her for a two-day sexual marathon that included sex at her house while her husband and kids were away. She was so into oral sex that, while giving me head, when I asked her to fuck me she refused until she'd sucked my cock some more. But she also said she loved to eat pussy and I would have liked to watch her do it while I fucked her from behind.

One thing I’ve learned about couples is that there is no one size fits all and communication is essential.

Feel free to check out my duo reviews if you're ever coming to LA. I share your pet peeve and that's why I'm always clear about who's playing with whom.

Quick update:
If you’re reviewing a provider and one is delisted, you can always to an “and friend” review.