Encore CS6 "DualLayer: This disc requires a layer break"
It looks like it is not possible to create a DVD DL with a ROM part that exceeds a layer's size.

So I created a 4 GB HD video (which also fits on a FAT32 file system) and created a DL disc with some GBs wasted space.

I tried to put another file onto the ROM part but this always fails as soon as the ROM part exceeds the layer limit, which is strange because DVD-ROM-only DL discs seem to work perfectly.