Mask shader on Android devices

Hi,

I’m currently working on a 2D game. For some feedback purpose I want to use a mask on a quad in order to make some parts of the quad transparents. To achieve this I use two shaders (one for the quad in background, and one on another quad to use it as mask).
Here they are:

The shader used on the background quad

Shader "FeedbackBG" {
	Properties {
		_MainTex("Base (RGB)", 2D) = "" {}
	}
	SubShader {
		Tags {"Queue" = "Geometry+10"} 
		Pass {
			Cull Off
			Lighting Off
			ZWrite On
			ZTest LEqual
            SetTexture [_MainTex] {}
        }
	}
	Fallback "Diffuse"
}

The one used on the mask

Shader "TexturedMask" {

	Properties {
		_Mask ("Culling Mask", 2D) = "white" {}
	}
		
	SubShader {
		Cull Off
 	  	Blend SrcAlpha OneMinusSrcAlpha
 		
		Tags {"Queue" = "Geometry+10"} 
 		Tags { "Queue" = "Transparent" }
		
		ColorMask 0
		ZWrite On
		ZTest Always
		Alphatest LEqual 0.5		
		
		Pass { 
			SetTexture[_Mask] {}
		}
	}
}

The combination of both works perfectly when I test it thanks to the webplayer but on Android devices (Samsung Galaxy S2 and Nexus7) only the background is displayed and the mask is not applied.
As I’m not really good in making shaders I wonder what I missed?

Any help would be appreciated.
Thank you.

I’ve finally put another camera to produce the mask effect.
As explained here : Treating an animated sprite as a mask? - Questions & Answers - Unity Discussions