Duotone is the generic name for multitone printing, which can be done with two, three or four inks.
Thanks in part to Photoshop, duotone is also a generic term for images using a color-separation printing scheme. Normally CMYK (Cyan, Magenta, Yellow, and Key (blacK)) separations, whereas a duotone would be two separations, and can be of any ink color (in offset printing).
for these images i have created duotones of the original prints on photshop. i have used spot colours to create them. this means that they can not be printed using C M Y K plates. as a spot colour has to be mixed separately and has a separate plate. in this case however the printing process will only need two to three plates anyway. so it would still work out cheaper than a cmyk print. however if you used a spot colour as well as cmyk it would be more expensive as there would be a 5 plated print.