I have a question: What single tanks are you currently using, and if they aren't HP130s or cave-filled LP121s or some other rather large tank, why go to doubles? That's a lot of complexity, change, and burden (on land or boat) for a few extra minutes of bottom time.
Also, all but the extremely minute of doubles can get you into serious deco on just about any recreational dive. For a 100-foot wreck dive using 32% I am bored and cold before I hit my NDL, and using my AL80 doubles I have over half of my backgas left when it comes time to ascend.
Even if you are doing shallow reef dives (20-40 ft) an HP120 or 130 will give even the hungriest of divers plenty of time to enjoy things, without the learning curve of a set of doubles. And don't forget, unless you are finding on screaming deal on a set of dubs, buying two new single tanks will be much cheaper than than buying a new set of doubles.
Peace,
Greg