I use Duotones to make great black and white digital ink jet prints. If you want to output your file to a professional 4-color printer, you may want to take a look at the color plates. Since a duotone has only one channel, you'll need to convert the file to Multichannel mode first.
Open a Duotone in Photoshop.
Choose Windows > Channels
The Duotone consists of a single channel.
Choose Image > Mode > Multichannel
The single color channel is converted to multiple spot color channels.
Select each individual channel to review its contents.
Choose Edit > Undo Multichannel to revert back to Duotone mode.