From "Ask JR" on the JR website:
What is double maduro and oscuro?
Double maduro is a wrapper which is a darker brown than a traditional maduro wrapper. Sometimes they can be almost black; they are also called "maduro maduro" or "oscuro" wrappers.
In error, some supposed experts have stated that it means that the cigar has a maduro binder and maduro wrapper; this is untrue. There is no such thing as a maduro binder, per se. Nobody wastes good wrapper leaf as binder, plus a good wrapper leaf could seldom function as a binder, due to strength and elasticity requirements.
Also, some quasi-experts have stated that it means that the cigar has two maduro wrappers on it... this is another bit of errant information.
Basically, leaves are bulked to be maduros, which typically means they begin the process as higher-primed and thicker leaves. They require the additional duration and increased fermentation temps in order to make the leaves smokable. The result is a darker colored leaf. However, due to the natural differences between the leaves, some become darker than others in the process of naturally fermenting them. They are then sorted into maduro and double maduro based on color.
Now sometimes you can tell that certain leaves, by their grain and heft, will end up being double maduro wrappers, so as they are bulking the pilons, they try to stack these in differing burros. But until the process is complete, you can not be certain as to the final color.
In short, double maduro is JUST a reference to color; not maduro binders and not double maduro wrappers, as I have also heard stated.