A PBR two-sided cloth shader that works correctly with Unity's lighting.

!Two-Sided Shader.md

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Two_Sided.shader

/// Created by @cortvi
Shader "Custom/Two sided"
{
	Properties
	{
		// You can learn everything about properties here:
		// https://docs.unity3d.com/Manual/SL-Properties.html
		_Color ("Main Color", Color) = (1,1,1,1)
		_MainTex ("Albedo (RGB)", 2D) = "white" {}
		_Cutoff ("Cutout threshold", Range(0.0,1.0)) = 0.1
		[NoScaleOffset] _SmoothnessMap ("Roughness map (A)", 2D) = "black" {}
		_Smoothness ("Flat roughness", Range(0.0, 1.0)) = 0.0
		[NoScaleOffset][Normal] _BumpMap ("Normal map (RGB)", 2D) = "bump" {}
		_BumpScale ("Normal scale", Float) = 1.0
	}

	SubShader
	{
		Tags { "Queue"="AlphaTest" "RenderType"="TransparentCutout" }

		CGINCLUDE
		// Whatever you write inside here
		// will be included in every pass
		struct Input
		{ float2 uv_MainTex; };

		fixed4 _Color;
		sampler2D _MainTex;
		sampler2D _BumpMap;
		float _BumpScale;
		sampler2D _SmoothnessMap;
		float _Smoothness;

		void surf (Input IN, inout SurfaceOutputStandard o)
		{
			fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
			fixed3 normal = UnpackScaleNormal (tex2D(_BumpMap, IN.uv_MainTex), _BumpScale).xyz;
			fixed smooth = tex2D (_SmoothnessMap, IN.uv_MainTex).a;

			// Feed surface output
			o.Albedo = c.rgb;
			o.Normal = normal;
			o.Metallic = 0.0;
			o.Smoothness = lerp(smooth, 1.0, _Smoothness);
			o.Alpha = c.a;
		}
		ENDCG
		
		Cull Front
		CGPROGRAM
		#pragma surface surf Standard fullforwardshadows vertex:vert alphatest:_Cutoff
		#pragma target 3.0

		void vert (inout appdata_full v)
		{
			v.normal = -v.normal;
		}
		ENDCG
		
		Cull Back
		CGPROGRAM
		#pragma surface surf Standard fullforwardshadows alphatest:_Cutoff
		#pragma target 3.0
		ENDCG
	}

	Fallback "Standard"
}

I can not tell you how much I would have appreciated a version with traditional frag/vert shaders. I HAVE to use them for what i'm doing (compute buffers don't work well with surf shaders), even if you cut out the lighting bit and just left the part that matters for double sided rendering.