|Zion, converted to B/W Tritone Image|
1. GO into adjustments and choose gradient map
2. GO image and convert to gray-scale and flatten
3. You should now be able to go into Image and choose Duotone and the dialog will look like this: be sure to convert to an 8 bit image.
4. Then you can use the Type dropdown and the custom to create duotone, tritone and quadtone images by mixing and matching to suit your taste.
5. Once finished convert back to 16 bit RGB for printing.
Give it a shot....