The barrel of the watch contains the mainspring, the wound piece of metal that provides the power for the watch to tick. Most watches get 36-40 hours of power from a fully wound mainspring. There are watches that have double barrels, and two mainsprings, to provide 80-100 hours of power. There are also some, more novelty watches, with several barrels and mainsprings.
Typically multiple barrels are placed in series to provide a more consistent power output over time.
And then there is Patek, with their 2013 patent on a single barrel with two stacked mainsprings. The main advantage Patek claims in their application is that by stacking them, you end up savings space. You can have a ratchet for winding on one side of the barrel, and a wheel on other for driving the movement, while still having the benefit of two springs.
I don't think this is particularly revolutionary, but it's an interesting space saving design.