how to apply multiple layers of textures to a billboard?

I have one texture, the bottom layer. And another, the top layer, which has cutouts through which i want the bottom layer to be seen. I also require the layers to be self illuminating.

How do I achieve this? I've attempted to apply both to a single mesh but i don't know how to layer them. I can make the top layer transparent and i can see the bottom layer through it, but i don't know how to make it transparent and self illuminating.

Thanks for any help

Using two materials for this is slow. Use an appropriate shader for performance and ease of use. It's really simple, because "self-illuminating" is a misnomer. It just means you don't use lighting, which yields a performance hit and requires more complex code.

Shader "2-Texture Alpha Blend" {

Properties {
    _MainTex ("Top Texture  (RGBA)", 2D) = ""
    _Texture2 ("Bottom Texture  (RGB)", 2D) = ""
}

SubShader {Pass {
    SetTexture[_Texture2]
    SetTexture[_MainTex] {Combine texture Lerp(texture) previous}           
}}

}