How to mix textures with multipass shader on Unity ?

How to mix textures with multipass shader on Unity ?

I want to mix textures with multipass shader on Unity as alpha-blending. But I don't know how to get the first pass's result with shader on Unity.

I've tried the variable "previous" and "primary", but they couldn't be used.

This is my shader.

 Shader "Unlit/MixTextures" {
     Properties {
       _MainTex ("Main Base(RGB) Alpha(A)", 2D) = "white" {}
       _MainTex2 ("Main2 Base(RGB) Alpha(A)", 2D) = "white" {}
     }
     SubShader {
         Tags{"Queue"="Transparent"}
         Pass{
             // Base        
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             
             sampler2D _MainTex;
                         
             struct v2f{
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
             };
             
             float4 _MainTex_ST;
             
             v2f vert(appdata_base v)
             {
                 v2f o;
                 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
                 return o;
             }
 
             float4 frag( v2f i ) : COLOR {
                 return tex2D(_MainTex, i.uv);
             }
 
             ENDCG
         }
         
         // pass 2
         Pass{
             // Add parts
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             
             sampler2D _MainTex2;
             float4 _TextureOffsetAndSize2;
                         
             struct v2f{
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float4 color : COLOR;
             };
             
             float4 _MainTex2_ST;
             
             v2f vert(appdata_base v)
             {
                 v2f o;
                 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.uv = TRANSFORM_TEX(v.texcoord, _MainTex2);
                 return o;
             }
 
             float4 frag( v2f i ) : COLOR {
                 
                 // * I want to use 1st pass result here * //
                 
                 float4 tex = tex2D(_MainTex2, i.uv);
                 tex.x *= tex.w;
                 tex.y *= tex.w;
                 tex.z *= tex.w;
                 return tex;
             }
             
             ENDCG
         }
     } 
     Fallback "Diffuse"
 }