To include HLSL code in other HLSLPROGRAM sections, you can use the HLSLINCLUDE directive.
Shader "Examples/ExampleShader"
{
    SubShader
    {
        HLSLINCLUDE
            // HLSL code that you want to share goes here
        ENDHLSL
        Pass
        {                
              Name "ExampleFirstPassName"
              Tags { "LightMode" = "ExampleLightModeTagValue" }
              // ShaderLab commands to set the render state go here
              HLSLPROGRAM
                // This HLSL shader program automatically includes the contents of the HLSLINCLUDE block above
                // HLSL shader code goes here
              ENDHLSL
        }
        Pass
        {                
              Name "ExampleSecondPassName"
              Tags { "LightMode" = "ExampleLightModeTagValue" }
              // ShaderLab commands to set the render state go here
              HLSLPROGRAM
                // This HLSL shader program automatically includes the contents of the HLSLINCLUDE block above
                // HLSL shader code goes here
              ENDHLSL
        }
    }
}