Looking for some advice or personal experience with aerodynamics of behind the seat hydration options. This year I used the x-lab dual sonic wing cage hydration carrier for my half iron distance races, but because I use a Torpedo between the arms, I only carried 1 bottle on the dual carrier in my HIM races as pictured from Florida 70.3. And for Olympic/sprints, I did not have the dual cage hydration carrier on the bike and only used the torpedo - (Although perhaps I should have kept the carrier and empty bottle(s) on back there for aero purposes?)
After visiting the X-lab website looking at the single carrier options, I see they have an aero pouch option which has a tool bag fitted around the bottle cage, which would work nicely for my needs. I asked the X-lab team which would be faster between the aero pouch single vs the dual carrier and they emailed back saying the dual carrier would be a few seconds faster over the HIM. This seems counterintuitive and I wanted to see what others think or have tested for single vs dual behind the seat options?
I just zip tie a cage under my saddle.
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Spoke with Sean on the convo you had with him - thanks for calling in BTW!
One bit that he didn’t mention…the data he relayed was a single bottle vs a dual bottle, not with the Aero Pouch considered - sorry about that! When you add the Aero Pouch it actually saves 13 seconds (from our testing), so from a single vs dual standpoint - generally speaking it is about the same. The time savings is also over a Full Distance, not half.
Note there are other factors that can alter the result, such as rider height/position, forward/rearward seating, angle of the cages/mounts (both carrier types have variable angles), even variance between dual carriers (ie Mini Wing vs new Aero Carbon Wing - the ACW will test faster).
Hope this helps! We will be expanding our Aerodynamic page once we launch our new website and we will be going more into detail about this as well! Right now it is a bit all over the place - sorry 
I’m no expert, but the air behind your bum will be so turbulent / disrupted, that it’s almost impossible to predict.
I doubt it’s any measurable difference, unless you go into an extreme: 1) two bottles side-to-side but wider than your hips, 2) sail-like bottle holder that Canyon Speedmax pros like to use, 3) 4-bottles sail combo that Magnus tried for Kona but they “only” allowed 3-bottles combo.
I don’t have any input on what’s faster but I do use the pouch vs a dual cage. I prefer the cleaner look and like that I can store my flat kit inside. I don’t have a draft box so storage is tricky. If I need more hydration, I simply dual the BTA. I also use a torpedo up front.
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Thank you for the clarification. I am glad to hear there is basically no difference between running the 2 options. I wanted to switch to the aero pouch/single bottle holder for simplicity as @Specialized22 mentions but only if there was no aero penalty.
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