Duotone (sometimes also known as Duplex) is a halftone reproduction of an image using the superimposition of one contrasting color halftone over another color halftone. This is most often used to bring out middle tones and highlights of an image.

Why might you use a duotone?

Duotones are ideal for two‑color print jobs with a spot color (such as a PANTONE Color) used for accent. Because duotones use different color inks to reproduce different gray levels, they are treated in Photoshop as single-channel, 8‑bit, grayscale images.

What is the advantage of creating a duotone from a gray scale image?

Duotones can be stark and arresting, and sometimes, that isn't what you want. In this case, a strip of duotone color sliced across a grayscale image can provide subtle colorization. Slanting the slice also gives your eye a vector to follow through the design.