Blended texture Shader

I'm trying to create a shader which displays a texture based on the slope of the vert normals on my mesh, I'm new to shader programming so I'm a little stuck. So far I have managed to calculate the slope and use it to "mask" out the MainTex.

The next stage is to draw into the masked area with _MainTex2 this is where I am stuck.

 Shader "SlopeShader" {
     Properties {
       _MainTex ("Texture", 2D) = "white" {}
       _MainTex2 ("Grass Texture", 2D) = "white" { }
       _Detail ("Detail", 2D) = "gray" {}
     }
     SubShader {
       Tags { "RenderType" = "Opaque" }
       CGPROGRAM
       #pragma surface surf Lambert vertex:vert
       
       struct Input {
           float2 uv_MainTex;
           float2 uv_MainTex2;
           //float2 uv_Detail;
           
           float slope;
       };
       
       sampler2D _MainTex;
       sampler2D _MainTex2;
       //sampler2D _Detail;
       
          void vert (inout appdata_full v, out Input o) {
             UNITY_INITIALIZE_OUTPUT(Input,o);
             o.slope = 1.0f - v.normal.y;
         }
       
       void surf (Input IN, inout SurfaceOutput o) {
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb * IN.slope;
           o.Alpha = IN.slope;
           //o.Normal = UnpackNormal (tex2D (_BumpMap, IN.uv_BumpMap));
       }
       ENDCG
     } 
     Fallback "Diffuse"
   }

In my attempt the black area is what I want the _MainTex2 to show up. Anyone know how I can achieve this?

Thanks, C.