What no-one seems to have commented is that using doubles will always make you have a higher SAC due to extra drag of the tanks the way they are no longer as streamlined and the extra regs. If you are also using a drysuit, then more drag and additional use of air to fill suit, especially if as per the profile you go up a bit and dump air and then down and add more.
Personally, my SAC goes from 10-10.5 on a single in a wetsuit to 12.5 using a single and pony and extra bits on a 40+m dive to 13.5 when using a drysuit on the same deep dive.