I'm not one to use RGB color more for Offset printing, and especially for duotones. If you work on RGB to create the perfect duotone, well.. that image will need to be converted to CMYK for printing. When it's converted, your (visual) 2 colors gets split across the CMYK channels and it's no longer a duotone technically. It may still look like 2 colors, but it's a 4 color image, not 2 color.
There are options for real duotones....
1 Standard Duotone
Start with a greyscale image:
Image > Mode > Duotone. This allows you to pick a second color:
If you want to adjust the amount of color you can alter the Duotone Curve for either color:
This creates a real duotone. Save as DCS 2.0 (or PSD) and place in a layout application.
2 Duotone with different pixel data per color
if you want different pixel data in a two-color image (still a duotone)...
Start with the same Greyscale image;
Convert the image to CMYK -
Image > Mode >CMYK . Look at the Channels Panel and highlight the channel with the most contrast (often the Y channel). Select all and
Edit > Copy. Then highlight the K channel and
Edit > Paste. Now fill the C, M, and Y channels with 100% white, so they are "empty".
You may then need to highlight the K channel and adjust Levels/Curves to bring the contrast back to normal:
Now, from the Channels Panel, choose New Spot Channel and pick your color:
Then, if you want you can copy the K channel and paste it on the Spot Color channel:
And any changes you make to either channel with alter that color specifically rather than the entire image:
(Leave the blank CMY channels there.)
Save as DCS2.0 (or PSD) for placement in a Layout Application.