Use Curves Twice?

If you mean the SweetFX curves shader, its not as simple to duplicate cause this is a shared shader. Unless you know your way around HLSL

However, if you still want to try, copy the curves content below the #if (USE_CUSTOM == 1) in the Custom.h.
For an easy start copy the required variables from the SweetFX.cfg also below #if (USE_CUSTOM == 1) and make sure to add an #undef <variableName> for each added #define <variableName> before the #endif.
Change return tex2D(RFX_backbufferColor, texcoord); in the PS_Custom function
to return CurvesPass( tex2D(RFX_backbufferColor, texcoord) );

Enabling Custom shader/effect in the Pipeline tab should now enable curves with the settings in the Custom.h.

Hope it works for you with the first try (did not try it myself its just out of the head)

Alternativly create the settings you would like in some tool like Photoshop, apply them to the LUT table in "ReShade\CustomFX\Textures" and use the so called "TUNINGPALETTE" shader instead to get the results in-game.

Crosire is absolutly right and since you seem already to know about how to use LUT.

If you are not familiar with photoshop I suggest the following.

Run the preview feature in the mediator and apply all color correction/contrast effects that you need plus one of the curve passes.

Now press L+prtscr to generate the LUT within a ReShade screenshot.

Disable all effects and only apply the just created LUT to the preview feature.

Now you have the choice to apply another curve pass and be done or even create another LUT to get rid of all color correction/contrast performance impact

EDIT: sorry, read your post too late, good you solved it the way its most efficient