This will allow you to duotone an image with a separate tint in the shadows and highlights.
Complete the image as for a highlight tint exactly as before.
Cnrl Click on the highlight adjustment layer.
Choose Duplicate Layer . . .
Enter Shadow Tint as a layer name..
Select the Mask icon for the new layer.
Go Image > Adjustments > Invert.
This will invert the mask to now mask the highlights instead of the shadows as previously.
The image will now have a tinted shadows and highlights.