Naturally, it's best to have multiple controllers and multiple channels. Wider and more is better. On the destination side, the recording drives better be of the same model and it's also better to use exactly same types of media from the same spindle case. But I've done some tests using various drives and various types of media on one instance of Nero or multiple instances of Nero... at various speeds from 1x to 8-12x, but never tried 16x since I'd had very few media that are good enough for 16x burning on most DVD burners.
If you have to burn to two DVD drives on one IDE, better connect 1:1 like this:
Primary Channel (Connector #1): first DVD burner (either Master or Slave)
Secondary Channel (Connector #2): second DVD burner (either Master or Slave)
It's good in that the source drive is located on SATA side which should help distribute the load a lot. First try at low speeds and try higher speeds later.