Alpha blending with more than one pass

I'm trying to create a shader which renders different parts of an object with different alpha depending on object coordinates. The following shader works, as long as I have only one pass (result not very good) but fails (no blending with background) when I have 2 passes:

This works:

 Shader "Custom/Bugtest" {
     Properties {
     }
     SubShader {
         Tags { "RenderType"="Transparent" "IgnoreProjector"="True" "Queue" = "Transparent" }
         LOD 200
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha      
         
         Cull off
         
         CGPROGRAM
         #pragma surface surf Lambert
         #pragma target 3.0
         #include "UnityCG.cginc"
         
         struct Input {
             float3 worldPos;
         };
         void surf (Input IN,inout SurfaceOutput o) {
             float4 objpos = mul( _World2Object, float4(IN.worldPos,1.0) );
             o.Albedo=float3(1,0.5,0.5);
             if (objpos.x>0)
                 o.Alpha=0.25;
             else
                 o.Alpha=1.0;
         }
         ENDCG
     } 
     FallBack "Transparent/VertexLit"
 }

But this not:

 Shader "Custom/Bugtest" {
     Properties {
     }
     SubShader {
         Tags { "RenderType"="Transparent" "IgnoreProjector"="True" "Queue" = "Transparent" }
         LOD 200
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha      
         
         Pass {
             Cull front    
         }
         Pass {
             Cull back
         }
         
         CGPROGRAM
         #pragma surface surf Lambert
         #pragma target 3.0
         #include "UnityCG.cginc"
         
         struct Input {
             float3 worldPos;
         };
         void surf (Input IN,inout SurfaceOutput o) {
             float4 objpos = mul( _World2Object, float4(IN.worldPos,1.0) );
             o.Albedo=float3(1,0.5,0.5);
             if (objpos.x>0)
                 o.Alpha=0.25;
             else
                 o.Alpha=1.0;
         }
         ENDCG
     } 
     FallBack "Transparent/VertexLit"
 }