What is the difference between single and dual annular combustor?
Here's an engine with old combustor cans:



Here's a single annular combustor:



And in cross section:



And now here's a dual annular combustor (DAC)  (pic from here) in cross section:



See how it's like one ring (or donut)  inside another ring. 

Wikipedia states "like an annular combustor, the DAC is a continuous ring without separate combustion zones around the radius. The difference is that the combustor has two combustion zones around the ring; a pilot zone and a main zone. The pilot zone acts like that of a single annular combustor, and is the only zone operating at low power levels. At high power levels, the main zone is used as well, increasing air and mass flow through the combustor."