"Double Layer DVD+RW is a form of DVD rewritable medium. Physically, it is constructed for fully random recording in the 32KB increments common to other DVD rewritable formats. Zero loss linking during ECC block recording allows streamed recording without skipping. A single logical format is defined for this medium: the Double Layer DVD+RW Basic Format. This document includes descriptions and command support double layer discs in both 80 mm and 120 mm".
Definitely recording in two layers of DVD+RW media is not a neglectable issue, although its marketing success could be questionable.
For more information visit the T10.org website and download the draft version here.
T10 is a Technical Committee of the InterNational Committee on Information Technology Standards (INCITS, pronounced "insights"). INCITS is accredited by, and operates under rules that are approved by, the American National Standards Institute (ANSI). These rules are designed to insure that voluntary standards are developed by the consensus of industry groups. INCITS develops Information Processing System standards, while ANSI approves the process under which they are developed and publishes them.