Here's a double-decker DVD burner from I-O Data that lets you copy a disk from one of its TEAC DV-W28SL burners to the other with no PC in sight. You can also plug this model DVR-UW8DP into your computer via USB 2.0, and use it as a normal pair of drives. You'll pay dearly for the privilege: at $448, but the thing looks rather cool with the two discs hanging halfway out and that swanky blue light underneath the power switch. Wait a couple of months and it'll probably cost a c-note.
External Dual DVD burner, the DVR-UW8DP [Akihabara News]