If memory serves me right, there was quite a lot of discussion on the topic of duos within the last year. If you search you should find the thread(s). It's not just that I'm lazy to search for you but I'm on slow-speed dial-up right now and it would take forever.
Definitely worth trying once but the general consensus was as above - generally not worth the price.
Unless the two ladies are truly bi and really, really into it most guys who've seen a few SPs tend to find it quite a letdown. This is probably because so many go in with images from porn videos in their head - only to discover that playing "safe" slows everything down.
For example, porn videos always show someone dipping their wick first in one lady and then immediately in another and then back in the first, and repeat (all uncovered of course). That just ain't gonna happen with any SPs who are safe (and these are the only ones you should contemplate doing duos with because it's easy to get distracted and suddenly find someone's not playing safe

). Instead of continuous action, there's an awkward break in the action each time you switch partners - because the condom needs to be changed each time you switch. Almost no ladies will allow you to use the same condom on multiple partners. Also, many ladies will insist on their toys being covered with condoms and so if you want to switch to use a toy from one to the other lady then you have to change the condom on the toy as well.
And although you might get ladies who DFK, and perhaps even DATY, you will generally be disappointed at how short that is and the fake moans and fake orgasms (oh if only us guys could give them an orgasm like that in only 2 minutes!).
In other words, nothing worse than a "duo" that's really nothing more than 2 SPs who are humouring you and just watching or faking some interest while waiting for you to switch partners.
Now, if the two ladies were both bisexual and really into each other and if both ladies had voracious sexual appetities then that's another situation entirely. We've read a few examples of really hot duos on the board and if someone's interested in a duo they'd be well-advised to go with a duo that's already been highly rated (if you search I think you'lll find several, but the only ones that come quickly to mind are Julia and Ingrid; Veronica and Shari; and Heidi and Ashly). Of course you also have to take into account YMMV in a big way - there are now 3 personalities to deal with and not just 2. Ladies who got along well on one occasion may not get along so well on the next occasion, and you may not hit it off with both.
IMHO duos of less than one hour are a waste of time since it hardly gives everyone time to get comfortable with each other. If you really want a great duo experience select the ladies carefully. It's usually best to select one lady you really like and are already comfortable with (i.e. you've seen her a number of times and she'd consider you a "regular") and then get her to invite the lady she most wants to play with. I'd also recommend considering multiple hours as that will give everyone time to relax and get comfortable (and allow repeat rounds).