The so called DualDisc is one of the proposed solutions to fill in the gap between the Compact Disc and the DVD age, as music industry and consumers have been quite slow in adapting to the next generation technology.
DualDisc essentially involves sticking a CD onto the back of the DVD Video or DVD Audio disc. The CD Audio layer is thinner than a Compact Disc, to ensure the DualDisc falls within the DVD specification thickness of 1.5mm.
DualDisc was devised as a way of adding backward compatibility to new audio disc format. For the labels, that allows them to ship a single product rather than separate DVD Audio and CD releases, and ensures consumers can build second-generation audio collections before they switch to new hardware.Source: The Register