Never!!!
As far as I know, dual-layer discs are made in separate layers and then glued together.
This is why all blank DVDs are single layer discs with a capacity of 4Gb. It wouldn't be possible for a home user to glue the layers together.
The studios would not want a recordable dual-layer format for fear of piracy. Since all movies are now dual-layer discs (9Gb) and the blanks are only single layer discs (4Gb), an identical one-to-one copy is not possible.