Edit2 - Solution inspired by @DMGregory

Shader "Sprites/Layered"
{
    Properties
    {
        [NoScaleOffset] _OutlineTex ("Outline", 2D) = "white" {}
        _OutlineColor ("Outline Color", Color) = (1,1,1,1) 
        [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
        [HideInInspector] [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        [HideInInspector] _Color ("Tint", Color) = (1,1,1,1)
        [HideInInspector] _RendererColor ("RendererColor", Color) = (1,1,1,1)
        [HideInInspector] _Flip ("Flip", Vector) = (1,1,1,1)
        [HideInInspector] [PerRendererData] _AlphaTex ("External Alpha", 2D) = "white" {}
        [HideInInspector] [PerRendererData] _EnableExternalAlpha ("Enable External Alpha", Float) = 0
    }

    SubShader
    {
        Tags
        {
            "Queue"="Transparent"
            "IgnoreProjector"="True"
            "RenderType"="Transparent"
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        Cull Off
        Lighting Off
        ZWrite Off
        Blend One OneMinusSrcAlpha

        Pass
        {
        CGPROGRAM
            #pragma vertex SpriteVert
            #pragma fragment LayerFrag
            #pragma target 2.0
            #pragma multi_compile_instancing
            #pragma multi_compile_local _ PIXELSNAP_ON
            #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
            #include "UnitySprites.cginc"

            sampler2D _OutlineTex;
            uniform fixed4 _OutlineColor;

        fixed4 LayerFrag(v2f IN) : SV_Target
        {
            fixed4 main = SampleSpriteTexture(IN.texcoord) * IN.color;
            main.rgb *= main.a;

            fixed4 layer = tex2D(_OutlineTex, IN.texcoord) * _OutlineColor;
            float alpha = layer.a;
            layer.rgb *= layer.a;  
            layer.a = 1;        

            return lerp(main, layer, alpha);
        } 
        ENDCG
        }
    }
}

Edit2 - Solution inspired by @DMGregory

Shader "Sprites/Layered"
{
    Properties
    {
        [NoScaleOffset] _OutlineTex ("Outline", 2D) = "white" {}
        _OutlineColor ("Outline Color", Color) = (1,1,1,1) 
        [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
        [HideInInspector] [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        [HideInInspector] _Color ("Tint", Color) = (1,1,1,1)
        [HideInInspector] _RendererColor ("RendererColor", Color) = (1,1,1,1)
        [HideInInspector] _Flip ("Flip", Vector) = (1,1,1,1)
        [HideInInspector] [PerRendererData] _AlphaTex ("External Alpha", 2D) = "white" {}
        [HideInInspector] [PerRendererData] _EnableExternalAlpha ("Enable External Alpha", Float) = 0
    }

    SubShader
    {
        Tags
        {
            "Queue"="Transparent"
            "IgnoreProjector"="True"
            "RenderType"="Transparent"
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        Cull Off
        Lighting Off
        ZWrite Off
        Blend One OneMinusSrcAlpha

        Pass
        {
        CGPROGRAM
            #pragma vertex SpriteVert
            #pragma fragment LayerFrag
            #pragma target 2.0
            #pragma multi_compile_instancing
            #pragma multi_compile_local _ PIXELSNAP_ON
            #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
            #include "UnitySprites.cginc"

            sampler2D _OutlineTex;
            uniform fixed4 _OutlineColor;

        fixed4 LayerFrag(v2f IN) : SV_Target
        {
            fixed4 main = SampleSpriteTexture(IN.texcoord) * IN.color;
            main.rgb *= main.a;

            fixed4 layer = tex2D(_OutlineTex, IN.texcoord) * _OutlineColor;
            float alpha = layer.a;
            layer.rgb *= layer.a;  
            layer.a = 1;        

            return lerp(main, layer, alpha);
        } 
        ENDCG
        }
    }
}

I have a hexagon sprite MainTex over which I want to render an outline texture SecondaryTex independent of the alpha value of the MainTex. Since I have a fairly large grid with many hexagons, two sprite renderers are not possible.

I have already achieved the overlay, but can’t get it to decouple from the MainTex alpha. Whenever I set the alpha of the sprite renderer to 0, the outline is also affected. I already tried to use a Maximum Node with the MainTex and the SecondaryTex alpha and use this value, but it doesn’t work.

How can I overlay and display the outline even if the MainTex / Sprite Renderer alpha is 0?

Edit:

This is the GameObject with the Sprite Renderer alpha set to 100%:

And this with the alpha set to 10%:

The alpha of the outline should not be influenced by the alpha of the MainTex sprite renderer, but it is. The same behavior occurs when I add a maximum node:

These are the import settings:

I have a hexagon sprite MainTex over which I want to render an outline texture SecondaryTex independent of the alpha value of the MainTex. Since I have a fairly large grid with many hexagons, two sprite renderers are not possible.

I have already achieved the overlay, but can’t get it to decouple from the MainTex alpha. Whenever I set the alpha of the sprite renderer to 0, the outline is also affected. I already tried to use a Maximum Node with the MainTex and the SecondaryTex alpha and use this value, but it doesn’t work.

How can I overlay and display the outline even if the MainTex / Sprite Renderer alpha is 0?

Edit:

This is the GameObject with the Sprite Renderer alpha set to 100%:

And this with the alpha set to 10%:

The alpha of the outline should not be influenced by the alpha of the MainTex sprite renderer, but it is. The same behavior occurs when I add a maximum node:

These are the import settings: