Transparent object VS itself

Shader issue, here. I tried to modify unity's toon shader a little bit to make transparent/translucent objects. The modification is just changing the render queue and type to 'Transparent' and add 'ZWrite On' and 'Blend SrcAlpha OneMinusSrcAlpha'. But from http://www.yourfilelink.com/get.php?fid=731204 and http://www.yourfilelink.com/get.php?fid=731205 you can see that it works well for objects on different meshes, but not the same mesh. I wonder why. Thanks. (The model is two semi-planes, each has a red triangle and the rest part is transparent).

I posted a similar question but didn't upload any image there.