This could be a big, big advantage for HD DVD. Toshiba announced that it has developed a three layer DVD that contains two HD DVD layers and one standard DVD layer. So DVDs utilizing this feature could contain one 4.78GB layer and a 30GB double layer HD DVD or two 8.57GB DVD layers with one 15GB HD DVD layer.
If studios decide to embrace this technology, it could do more to help proliferate HD DVD than any other single factor. By allowing users to build an HD DVD collection before actually investing in an HD DVD player, I believe it would serve as a heavy motivator when the user finally decides to update. This would make a lot of sense for studios and manufacturers a like, but as Charlie White pointed out at Gizmodo, studios vastly prefer to sell the same content over and over.