Using stencil buffer to make windows

Question

I would like to create windows in different walls and I want it to be dynamic so I do not want to change geometry of the wall because I want to be able to move the window. So I want to use stencil shader. I am new to shaders, but I have found some things already and I have come up with this:

Shader I use for Windows:

Shader "Custom/Window" {
Properties {
    _MainTex ("Base (RGB)", 2D) = "white" {}
    _StencilVal ("stencilVal", Int) = 1
}
SubShader {
    Tags { "RenderType"="Opaque" }
    LOD 200

    ZWrite Off
     ColorMask 0

     Pass {
         Stencil {
             Ref [_StencilVal]
             Comp NotEqual
             Pass replace
         }
     }

    CGPROGRAM
    #pragma surface surf Lambert

    sampler2D _MainTex;

    struct Input {
        float2 uv_MainTex;
    };

    void surf (Input IN, inout SurfaceOutput o) {
        half4 c = tex2D (_MainTex, IN.uv_MainTex);
        o.Albedo = c.rgb;
        o.Alpha = c.a;
    }
    ENDCG
}
FallBack "Diffuse"

}

Shader I use for walls:

Shader "Custom/Rest" {
Properties {
    _MainTex ("Base (RGB)", 2D) = "white" {}
    _Color ("Color", Color) = (1, 0, 0, 1)
    _StencilVal ("stencilVal", Int) = 1
}
SubShader {
    Tags { "RenderType"="Opaque" }
    LOD 200

    Stencil {
        Ref [_StencilVal]
        Comp NotEqual
    }

    CGPROGRAM
    #pragma surface surf Lambert

    sampler2D _MainTex;
    uniform fixed4 _Color;

    struct Input {
        float2 uv_MainTex;
    };

    void surf (Input IN, inout SurfaceOutput o) {
        half4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
        o.Albedo = c.rgb;
        o.Alpha = c.a;
    }
    ENDCG
}
FallBack "Diffuse"

}

I set _StencilVal from script so that every wall has its own unique value and I have achieved pretty good result but the problem is that I can not look through two (or more) windows at the same time. When they overlap in the view the second window is grey (it is not seethrough) but I can still look through the first window.