http://www.afterdawn.com/news/archive/7951.cfm
According to the patent, which several top Warner executives as the inventors, "a dual disc may also be formed with two high-capacity data layers, one conforming to the HD DVD format and the other conforming to the BD [Blu-ray disc] format."
Of course, there are downsides to this method. The main downside would be that each HD format would only have one layer on the disc, that's 25GB Blu-ray data and 15GB HD DVD. Then when you consider the capacity that HD content demands, studios would find themselves having to release some movie titles with multiple discs just like DVD.
