I'm going to fulfill the fantasy of having a duo my next Vegas vacation.  I usually go 90 minutes for a singles session, is 90 minutes definitely the way to go for a duo or can I get away with one hour?  The doubled price is a factor plus I'd like to have a separate singles session later in my trip.  

I've always done 90 for duos.  The last thing you want to happen is to have the clock run out while your in the midst of an extra special session.

Of course this depends on your funds, but the extra 30 minutes can really enhance your session. People are human and can run a bit late, so the extra time can help that way, but if you are not experienced with group play, as it sounds like you are not, it's just a totally different dynamic. That extra person in the room creates more opportunity for chit chat, and that can lessen your play time.  
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See both for 90, and make sure they have played together before as that is a big help. Hopefully, you have access to reviews of them in a duo (or get a great recco) as that can tell you how interactive the two girls will be as that is extremely important to most gents, including myself. Good luck and enjoy!

If you're looking for girl-girl action, plus double attention to you, plus multiple entries, plus multiple pops, then by all means, go for 90 minutes.  I'm at the age where watching the ladies together for 30 minutes and then jumping in for the last 30 is enough for me. I booked 90 minutes when I had 3 ladies at a time; allowed for lots of interesting configurations.  But if you have the stamina and lots of different things you want out of the session, by all means go with 90.

As one half of a dynamic duo, (Hi Rob 2 or 3 hours is perfect. If budget is a factor, 90 mins minimum.  

First, to answer your direct question, one hour is never enough time for a duo, IMO.  The variable is whether or not this is going to be two girls tag-teaming YOU, or whether you are expecting both girl-girl and three-way action during the session, in which case, if it's just a tag-team, 90 minutes should be enough, but if you are expecting more, my advice is to consider two hours.  Time flies when you're having fun, so go big or go home.  

Last thing you want is to feel you wasted your time or money.  Get the value you want.  Or talk to the girls about the options of going longer.  

I have done duos where the ladies were both mostly pleasing me and not each other. In this case, an hour was plenty.

I can certainly vouch firsthand that an hour is not nearly enough time to play with you and the very wild Ms. Brielle James. Quite a bit has gone on with the three of our entangled, fully engaged bodies, on multiple occasions now, and some of those deliciously decadent things are better off not rushed...so we are in complete agreement yet again Ms Cruz.

...you will likely see several beautiful women responding.  If's a pretty common request and many awesome gals will give you a great experience.  Contrary to what some self appointed experts have said, several will offer incall for this and I suggest that as a great idea for one of the ladies to host for a couple reasons.   She will be more comfortable and the gals she may recommend will likely be familiar with her place as well.  Def do the 90 minutes!  If you want to PM me, I can recommend a gal that will rock your world and provide the real deal with a bisexual partner that will do the same.

One hour goes by super fast. The last appointment we had was multiple hours. I suggest 2 hours if you can. It takes a little longer to get to know each other. Sharing a bottle is always a great way to break the ice. Cheers! 🥂

You could consider having both of them help you plan, just asking what works best for the three of you. Easiest way could be contacting one and asking her for a friend.

If you're going for the three-way, I think it's a good approach to discuss with both escorts just be on an informal stance to get the best value out your time and money

Right now, my plan is to meet one girl my first night of my Vegas vacation and discuss who she would like to do a duo with later on in my trip.  Definitely sounds like 90 minutes will be the minimum.  2 hours with 2 girls is certainly not in the budget.  

My plan now is to meet a girl (I already have picked out) on my first night in town and discuss a duo later on during my trip.  I think this is the best option so we can agree on a third girl and talk about what I expect.  

You are wise to get as much as possible out on the table before the session starts. The comfort of the two ladies with each other--and with what you want to watch them do--is the sine qua non of a good duo soiree.

Did a threesome in Vegas a couple years ago.  Was an evening appointment for two hours.  The ladies stayed for almost three hours.  No extra charge.

If your Super Human like me and can come twice in an hour, then maybe an hour is enough

90  minutes  is  the  minimum,  two  hours  is  better.
The  girl  wouldn't  be  offended;  your  fantasy,  your  money!

In VEGAS! Don't go poor boy on this. Balls out,man
Do the 2 hours, find two girls who are Bi and that would be into each other. Make sure they're FC friendly, or...???
Gotta say, I like your style, "I'm playing around with one girl for half an hour then the other girl walks in to join us"
DO IT! YOLO  

Some refer to it as "premature ejaculation."