Everyone's personal experience varies, but I would imagine that at least part of the reason that many people tend to get more vapor/flavor from duals-vs-singles is the basic fact that duals have more surface area and contact with
juice, and if all other factors are equal, that would result in more vapor being produced.
It's important to point out the "all other factors being equal" portion of my statement. With variable wattage/voltage, a single can produce as much (or more) vapor than someone else running duals in a non-altered rig. Once voltage/wattage are altered, vapor production is more a function of power than of coil surface area.