You cannot mix a Surface shader ('closure') with a RGBA shader such as Flat, otherwise, the result of the Surface shader is lost. You need to use two shaders of the same type such as Standard Surface.

Mode

Mix Weight

Shader 1

Shader 2

 

Shading network used in above example

 

 

Workflow Example

Animated Facing Ratio shader -> Mix Weight. Snow shader -> shader1, Mountain shader -> shader2.