One bad news. A lot of those images you are linking are not duotones... they are tritones. One Blue, yellow and black.
But let's see the color separations.
Using only one grayscale image is not good enough.
Take a look at this two posts:
Preparing design for duotone printing?
Printing photographs when job is a 2 spot color job
The basic idea is: take one "pure" channel, any of the RGB ones and use it as an ink. Use the opposite color.
On those two links I posted, I use one grayscale image as a base and one pure channel to do the "coloring".
But as you do not have a yellow channel I combined the Red and Green channel with multiply at the same time and used it as Blue. Then I used the blue as the yellow.
You also can tweak your CMYK color profile to reduce the amount of black used on the conversion.
Go to Edit>Color adjustments (May+Ctrl+K) And in the CMYK profile choose the custom profile. PLay with the settings there.
Then you can replace one ink with another color using multichannels.