it is not that dificult, but you need all the players (more politics then anything else) to play nice

When CD was created it was 1.2mm thick and the data was on the opposite side of the disk. The laser passed through the 1.2mm of plastic to read the data. When DVD was created the data was moved to .6mm, in theory it could have bueen 1/2 as thick but the thinner a disk the more fragile it becomes, so it became two bonded .6mm disks. BD has the data at .1mm. You can in theory have .1mmBD and on the same side at.6mm the HD DVD or BD, but even simpler is to make a BD that is half as thick (.6mm) and stick it on the back of a 1/2 DVD or HD DVD.

The original DVD was at SL DS DVD-10(though people don’t like DS) because all you did was make two independent 1/2 disks that were glued together. Then they found out that you could make it to read through the first layer and read the second. So disks made the same way but meant to read both 1/2 disks through the same layer were created. The first DL where created the other more complicated way and that lived on in DLDS (DVD-18) .

A BD/HD DVD (or DVD) could be made in several ways

1)make a 1.1 HD DVD (or DVD) and then add the .1 BD on top
2)make a .6mm BD and ½ a DVD-18 and glue them together
3)make a .5mm HD DVD and add a .1 BD and glue it to a .6mm SL HD DVD