Creating and Editing Duotones in CorelDRAW
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.