How to join textures from multiple quads on another quad?

I want to draw two times on two quads and then join it on the screen (or other quad?) - one on top of the first, like background parts first (with some postprocess), and then objects (with different postprocess) on top of it and in the end apply postprocess lighting on the combined image (background with postprocess+objects with postprocess), but at this point I don’t know how I can join two quads- could someone point out the direction I should take?(edytowane)

For now:

  1. I am drawing to few render targets
  2. Drawing to one quad few of them
  3. Drawing to another quad the rest
  4. How to join above quads?(edytowane)