Here we see the Adobe PageMaker 'Colour' Print dialogue window and the Colour Palette. The resulting separations will include the separated duotone plates as well as any other colour plates included in the job. Duotone, Tritone etc. Usually one of the colours will be made to dominate the other although of course an even mix of each can be used. A classic example used and seen by me over many years is one where a blue and black might be the two spot colours used to print a publication.
Any picture subjects like stainless steel, aluminium etc.
Using a selection, the black could also be reduced in any sky areas. Not relevant to duotones but, in high quality publications included black and white pictures can look a little special if produced as CMYK images although the stock and press need to be of good quality too to avoid colour shifts.