I'll try to be as clear as I can to be easily understood.

I know that by means of certain variables passed from vs to fs values are interpolated so each fragment gets the appropriate ones (like the fragment coords).
I want to take advantage of this "free" interpolation and input a second set of 4 vertices in vs for a second texture so that in fs I can mix texture[0] with a displaced (projected) texture[1] just by specifying texCoord[0] and (ready interpolated)texCoord[1].

1. Is it possible in the first place?
2. Does GL know to match current fragment coords to the respective coords of the second set of vertices?
3. In what order are the vertices to be specified to match?
4. What qualifiers can the vertices be? (attributes, varyings) Can you give an example?

What steps do I have to do? Where can I read about them?