“In March 2006, the DVD+RW Alliance approved DVD+RW part 2: Dual Layer, volume 1; DVD+RW 8.5 Gbytes, Basic Format Specifications, version 1.0, March 2006, which has been released for distribution. As of 2009, no manufacturers currently produce any media, or drives that support the format.”
A similar scenario applies to the DVD-RW DL format. It has been approved and JVC have discs but they have been put on hold due to the cost. Dual layer RW blu-ray discs are available and this has probably given the companies less incentive to provide any dvd±rw DL hardware/media to the consumers.