Using an object as a mask for another object (using Spine)

Hi! I'm doing a project and am using Spine for animations. I export those to Unity and use the Spine-runtime to control everything in Unity.

I wanted to use one animated object as a mask for another animated object. I know next to nothing about shaders. I only use the shadergraph here and there.

Spine objects come with their own shaders and I tried to modify a bit the code to achieve what I want, and the second object is indeed masked within the first object, but the problem is that it is masked in the whole image area, even where pixels are trasparent.

Here's a gif with the behaviour right now: https://i.gyazo.com/d7bd76bfd662da1e1afb0aaf1a7bb8d6.mp4

Here are the shader codes.

Mask

 Shader "Spine/SkeletonMask" {
     Properties {
         _Cutoff ("Shadow alpha cutoff", Range(0,1)) = 0.1
         [NoScaleOffset] _MainTex ("Main Texture", 2D) = "black" {}
         [Toggle(_STRAIGHT_ALPHA_INPUT)] _StraightAlphaInput("Straight Alpha Texture", Int) = 0
         [HideInInspector] _StencilRef("Stencil Reference", Float) = 1.0
         [HideInInspector][Enum(UnityEngine.Rendering.CompareFunction)] _StencilComp("Stencil Comparison", Float) = 8 // Set to Always as default
 
         // Outline properties are drawn via custom editor.
         [HideInInspector] _OutlineWidth("Outline Width", Range(0,8)) = 3.0
         [HideInInspector] _OutlineColor("Outline Color", Color) = (1,1,0,1)
         [HideInInspector] _OutlineReferenceTexWidth("Reference Texture Width", Int) = 1024
         [HideInInspector] _ThresholdEnd("Outline Threshold", Range(0,1)) = 0.25
         [HideInInspector] _OutlineSmoothness("Outline Smoothness", Range(0,1)) = 1.0
         [HideInInspector][MaterialToggle(_USE8NEIGHBOURHOOD_ON)] _Use8Neighbourhood("Sample 8 Neighbours", Float) = 1
         [HideInInspector] _OutlineMipLevel("Outline Mip Level", Range(0,3)) = 0
     }
 
     SubShader {
         Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" "PreviewType"="Plane" }
 
         Fog { Mode Off }
         Cull Off
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha
         Lighting Off
 
         Stencil {
             Ref[_StencilRef]
             Comp[_StencilComp]
             Pass Replace
         }
 
         Pass {
             Name "Normal"
 
             CGPROGRAM
             #pragma shader_feature _ _STRAIGHT_ALPHA_INPUT
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             sampler2D _MainTex;
 
             struct VertexInput {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
                 float4 vertexColor : COLOR;
             };
 
             struct VertexOutput {
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float4 vertexColor : COLOR;
             };
 
             VertexOutput vert (VertexInput v) {
                 VertexOutput o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.uv = v.uv;
                 o.vertexColor = v.vertexColor;
                 return o;
             }
 
             float4 frag (VertexOutput i) : SV_Target {
                 float4 texColor = tex2D(_MainTex, i.uv);
 
                 #if defined(_STRAIGHT_ALPHA_INPUT)
                 texColor.rgb *= texColor.a;
                 #endif
 
                 return (texColor * i.vertexColor);
             }
 
             ENDCG
         }
 
         Pass {
             Name "Caster"
             Tags { "LightMode"="ShadowCaster" }
             Offset 1, 1
             ZWrite On
             ZTest LEqual
 
             Fog { Mode Off }
             Cull Off
             Lighting Off
 
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_shadowcaster
             #pragma fragmentoption ARB_precision_hint_fastest
             #include "UnityCG.cginc"
             sampler2D _MainTex;
             fixed _Cutoff;
 
             struct VertexOutput {
                 V2F_SHADOW_CASTER;
                 float4 uvAndAlpha : TEXCOORD1;
             };
 
             VertexOutput vert (appdata_base v, float4 vertexColor : COLOR) {
                 VertexOutput o;
                 o.uvAndAlpha = v.texcoord;
                 o.uvAndAlpha.a = vertexColor.a;
                 TRANSFER_SHADOW_CASTER(o)
                 return o;
             }
 
             float4 frag (VertexOutput i) : SV_Target {
                 fixed4 texcol = tex2D(_MainTex, i.uvAndAlpha.xy);
                 clip(texcol.a * i.uvAndAlpha.a - _Cutoff);
                 SHADOW_CASTER_FRAGMENT(i)
             }
             ENDCG
         }
     }
     CustomEditor "SpineShaderWithOutlineGUI"
 }

Masked:

 // Spine/Skeleton PMA Screen
 // - single color multiply tint
 // - unlit
 // - Premultiplied alpha Multiply blending
 // - No depth, no backface culling, no fog.
 // - ShadowCaster pass
 
 Shader "Spine/Blend Modes/Skeleton PMA Screen Masked" {
     Properties {
         _Color ("Tint Color", Color) = (1,1,1,1)
         [NoScaleOffset] _MainTex ("MainTex", 2D) = "black" {}
         [Toggle(_STRAIGHT_ALPHA_INPUT)] _StraightAlphaInput("Straight Alpha Texture", Int) = 0
         _Cutoff ("Shadow alpha cutoff", Range(0,1)) = 0.1
         [HideInInspector] _StencilRef("Stencil Reference", Float) = 1.0
         [HideInInspector][Enum(UnityEngine.Rendering.CompareFunction)] _StencilComp("Stencil Comparison", Float) = 8 // Set to Always as default
 
         // Outline properties are drawn via custom editor.
         [HideInInspector] _OutlineWidth("Outline Width", Range(0,8)) = 3.0
         [HideInInspector] _OutlineColor("Outline Color", Color) = (1,1,0,1)
         [HideInInspector] _OutlineReferenceTexWidth("Reference Texture Width", Int) = 1024
         [HideInInspector] _ThresholdEnd("Outline Threshold", Range(0,1)) = 0.25
         [HideInInspector] _OutlineSmoothness("Outline Smoothness", Range(0,1)) = 1.0
         [HideInInspector][MaterialToggle(_USE8NEIGHBOURHOOD_ON)] _Use8Neighbourhood("Sample 8 Neighbours", Float) = 1
         [HideInInspector] _OutlineMipLevel("Outline Mip Level", Range(0,3)) = 0
     }
 
     SubShader {
         Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
         LOD 100
 
         Fog { Mode Off }
         Cull Off
         ZWrite Off
         Blend One OneMinusSrcColor
         Lighting Off
 
         Stencil {
             Ref[_StencilRef]
             Comp[_StencilComp]
             //Pass Keep
         }
 
         Pass {
             Name "Normal"
 
             CGPROGRAM
             #pragma shader_feature _ _STRAIGHT_ALPHA_INPUT
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             uniform sampler2D _MainTex;
             uniform float4 _Color;
 
             struct VertexInput {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
                 float4 vertexColor : COLOR;
             };
 
             struct VertexOutput {
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float4 vertexColor : COLOR;
             };
 
             VertexOutput vert (VertexInput v) {
                 VertexOutput o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.uv = v.uv;
                 o.vertexColor = v.vertexColor * float4(_Color.rgb * _Color.a, _Color.a); // Combine a PMA version of _Color with vertexColor.
                 return o;
             }
 
             float4 frag (VertexOutput i) : SV_Target {
                 float4 texColor = tex2D(_MainTex, i.uv);
 
                 #if defined(_STRAIGHT_ALPHA_INPUT)
                 texColor.rgb *= texColor.a;
                 #endif
 
                 return (texColor * i.vertexColor);
             }
 
             ENDCG
         }
 
         Pass {
             Name "Caster"
             Tags { "LightMode"="ShadowCaster" }
             Offset 1, 1
 
             ZWrite On
             ZTest LEqual
 
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_shadowcaster
             #pragma fragmentoption ARB_precision_hint_fastest
             #include "UnityCG.cginc"
             struct v2f {
                 V2F_SHADOW_CASTER;
                 float4 uvAndAlpha : TEXCOORD1;
             };
 
             uniform float4 _MainTex_ST;
 
             v2f vert (appdata_base v, float4 vertexColor : COLOR) {
                 v2f o;
                 TRANSFER_SHADOW_CASTER(o)
                 o.uvAndAlpha.xy = TRANSFORM_TEX(v.texcoord, _MainTex);
                 o.uvAndAlpha.z = 0;
                 o.uvAndAlpha.a = vertexColor.a;
                 return o;
             }
 
             uniform sampler2D _MainTex;
             uniform fixed _Cutoff;
 
             float4 frag (v2f i) : SV_Target {
                 fixed4 texcol = tex2D(_MainTex, i.uvAndAlpha.xy);
                 clip(texcol.a * i.uvAndAlpha.a - _Cutoff);
                 SHADOW_CASTER_FRAGMENT(i)
             }
 
             ENDCG
         }
     }
     CustomEditor "SpineShaderWithOutlineGUI"
 }