Shader to apply Alpha to other images

I am in need of a shader for my iOS/Android 2D game. I searched for a few hours but had no luck. The shader needs to:

Make all textures within N unity units in depth from the given image use the alpha from the image. It has to be within a certain Z depth so that an image that is farther away behind it will still be visible. I can't have a shader with multiple texture inputs because I am using some sprite sheets and I have no clue how to change the sprite sheet system to work with that.

I will be using this shader to make a texture (example: Grass) fade into another texture (example: Lava).

My knowledge of how to make shaders is very limited, so I don't know how to write it myself. Hopefully the picture and explanation are sufficient. Thanks!