According to Wood Reserve they age the whisky in a normal charred oak barrel for part of its life, then switch it too a "roasted" charred barrel to finish ageing. Double barrel whisky usually just means they mix the whisky together. Distillers dump all the whisky that matured that year (say 1,000 barrels at a time), into a huge vat, and bottle it. I guess Wood Reserve does this at half life, and instead of bottling it, puts it in a new barrel to finish its age.
In contrast, a single barrel whisky means when the whisky comes of age, they bottle the whisky one barrel at a time. Most whisky is double barrel and this is not to be confused with single and double malt.
The double barrel process is cheaper to bottle and allows for constancy in taste.
I dont like whisky, but its still an interesting process. Here is an video of how they taste test the whisky, a little whisky history, and some Dalmore 64 (currently estimated at around $122,000 a bottle):
Edit: changed my use of the term blended into mixed, as blended is more oft used in reguards to malt.