In Photoshop, open the image you chose to work with, then go to the adjustments panel and create a new posterize adjustment layer. You can explore other adjustment layer options such as a threshold that may work better depending on your image.
Move the lever until you get some nice contrast (we used a value of 4).
There are a couple of ways you can use to create the duotone, with a solid color adjustment layer and blending modes or with a gradient map adjustment layer. We will illustrate both ways.
Option 1: Adjustment layers and blending modes
Step 1: Create a color fill adjustment layer
Create a new color fill adjustment layer by clicking on the small black and white circle on the bottom of the layers panel and choose solid color.
Choose the shade you want to use on the solid color picker (we used #2a95a8).
Step 2: Adjust the color with blending modes
Then, change the blending mode to multiply to fill the white portions of the original image with the selected color.
If you use screen instead you will fill the black portions with the selected color.
You can also check out how other blending modes work on your image. Like hard light.
To change the fill color, double-click on top of the solid color fill thumbnail (we used #a8a42a).
Option 2: Gradient map adjustment layer
Step 1: Create a gradient map adjustment layer
To create a duotone with a gradient map adjustment layer head to the adjustments panel and choose that option.
Step 2: Select your gradient
Click on top of the gradient on the properties panel to open the gradient editor and choose one of the presets or create a custom gradient yourself.
You can also check the reverse box to reverse the gradient.
Check out all the different ways you can use gradient maps: