Shader Question about Alpha

I want to make a shader that uses two passes to render an object. I want the results from the second pass to be rendered so that they are slightly transparent(some alpha), where that alpha can be varied by a slider in the unity editor.

Please suggest how to do this.

Current Shader:

 Shader "Examples/2 Alpha Blended Textures" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         //_BlendTex ("Alpha Blended (RGBA) ", 2D) = "white" {}
     }
     SubShader {
         Pass {
             Cull Front
             
             // Apply base texture
             SetTexture [_MainTex] {
                 combine texture
             }
         }
 
         
         
         
         Pass {
             Cull Back
             
             // Apply base texture
             SetTexture [_MainTex] {
                 combine texture
             }
         }
 
 
 
 
     }
 
     FallBack "Diffuse"
 }