A duotone image allows you to assign from one to four spot color inks to images. With an image selected, choose Bitmap | Mode | Duotone to bring up the Duotone dialog box where you can choose the number of inks and assign them. Once you’ve got the image just right, click OK and the duotone is created.
What seems baffling is that there is no way to edit the images. Or at least it may seem that way. With the image selected, use the same command to bring the dialog box back up with the current settings. Edit them as much as you like and click OK again to return to the drawing page. So while there is no obvious way to edit the files, you certainly can edit them in the same way they were initially created.