Shader "cool shader" {
Properties {
    ...
}
SubShader {

    CGPROGRAM
    // need to declare vertexOutput somewhow here
    float4 sharedFragFoo(vertexOutput i) : COLOR  // How to make smth like this?
    {
        ....
        return float4(...);
    }
    ENDCG

    pass {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        vertexOutput vert(vertexInput v) {
            vertexOutput o;
            ...
            return o;
        }

        float4 frag(vertexOutput i) : COLOR
        {
            return sharedFragFoo(i); // call the shared between passes function
        }

        ENDCG
    }

    pass {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        vertexOutput vert(vertexInput v) {
            vertexOutput o;
            ...
            return o;
        }

        float4 frag(vertexOutput i) : COLOR
        {
            return sharedFragFoo(i); // call the shared between passes function
        }

        ENDCG
    }
}
}

Shader "YourShader"
{
    ...

    CGINCLUDE

    #include "UnityCG.cginc"

    struct shared_v2f
    {
        float4 pos : SV_POSITION;
    }

    shared_v2f myVert( appdate_img v )
    {
        shared_v2f o;

        o.pos = mul (UNITY_MATRIX_MVP, v.vertex);

        return o;
    }

    fixed4 myFrag( shared_v2f i ) : SV_Target
    {
        return fixed4( 1.0, 0.5, 0.0, 1.0 );
    }

    ENDCG

    SubShader
    {
        ...

        Pass
        {
            CGPROGRAM

            #pragma vertex myVert
            #pragma fragment myFrag

            ENDCG
        }
    }
}

Shader "cool shader" {
    Properties {
        // properties
    }
    SubShader {
        ...

        pass {
            Cull Front
            ZWrite Off
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "shared.cginc"
            ENDCG
        }

        pass {
            Cull Back
            ZWrite Off
            Blend SrcAlpha OneMinusSrcAlpha
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "shared.cginc"
            ENDCG
        }
    }
}

#ifndef SHARED_FOO
#define SHARED_FOO

uniform sampler2D _MainTex;
uniform float4 _MainTex_ST;
uniform float4 _Color;
// other variables....

struct vertexInput {
    float4 vertex : POSITION;
    float3 normal : NORMAL;
    float4 texcoord : TEXCOORD0;
};

struct vertexOutput {
    float4 pos : SV_POSITION;
    float4 tex : TEXCOORD0;
    float4 posWorld : TEXCOORD1;
    float4 posInObjectCoords : TEXCOORD2;
    float3 normalDir : TEXCOORD3;
};

vertexOutput vert(vertexInput v) {
    vertexOutput o;
    // do staff
    return o;
}

float4 frag(vertexOutput i) : COLOR
{
    // do staff
    return float4(...);
}
#endif // SHARED_FOO

Shader "Custom/YourShader"
{

Properties
{
    _MainTex("Texture", 2D) = "white" {}
}

CGINCLUDE
//This code is available to all passes in your shader below

    sampler2D _MainTex;
    float4 _MainTex_TexelSize;

    struct appdata
    {
        float4 vertex : POSITION;
        float2 uv : TEXCOORD0;
    };

    struct v2f
    {
        float2 uv : TEXCOORD0;
        float4 vertex : SV_POSITION;
    };

    float4 aSamplingFunction(sampler2D tex, float2 uv, float4 size)
    {
        return SOMETHING;
    }

ENDCG

SubShader
{
    Cull On
    ZWrite Off
    ZTest LEqual

    Pass// #0
    {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        #include "UnityCG.cginc"

        v2f vert(appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.uv = v.uv;
            return o;
        }

        float4 frag(v2f i) : SV_Target
        {
            float4 col = aSamplingFunction(_MainTex, i.uv, _MainTex_TexelSize);
            return col;
        }
        ENDCG
    }

    Pass// #1
    {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        #include "UnityCG.cginc"

        v2f vert(appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.uv = v.uv;
            return o;
        }

        float4 frag(v2f i) : SV_Target
        {
            float4 col = aSamplingFunction(_MainTex, i.uv, _MainTex_TexelSize);
            return col;
        }
            ENDCG
        }
    }
}