Brush a texture with another texture

I am trying to brush a texture with another texture. I think the best way to achieve this is using a shader that takes a main texture and also a brush texture. But I am new with shaders and I don't know how to do this. I search other threads but I couldn't find the solution..

Also how can I pass a position input to a shader? I looked at many shaders but I didn't see any shaders that takes an input from outside.

Thanks