Normally, a dual-layer disc stores either 9.4GB in the case of DVD (4.7GB per layer) or 30GB in the case of HD-DVD (15GB per layer). The new disc has a layer of regular DVD and a layer of HD-DVD. Presumably, the same content would be put on each layer, allowing the disc to be played in either a DVD or HD-DVD player.
I'm not sure how succesful this will be, however. Purely anecdotal evidence (in the form of movies I've recently purchased) indicates that quite a few new releases are using both layers of a dual-layer disc for regular DVD content, be it both widescreen and pan and scan versions of the film, or ludicrous amounts of deleted scenes and extras.
The new format would obviously cut the amount of available space in half, and I'm not sure either consumers or content producers would go for that. Consumers likely wouldn't want to spend money on a stripped-down version of a DVD when the one next to it on the shelf has a lot more goodies. Content providers, on the other hand, probably wouldn't be too jazzed about producing another version of the same title.