I have two materials that I'm trying to bake into one texture. I have the same image texture node selected on each material, but when I bake it, only one material shows up.

First material with image texture node:
[![Pebbles material node][1]][1]

Second material with the same image texture node selected:
[![Ground material node][2]][2]

Here's the bake result. Only the second material shows up.
[![Bake result][3]][3]

[EDIT] 

Result of unwrapping with Smart UV Project:
[![enter image description here][4]][4]

I'm looking for something like this where the materials overlap in a way consistent with how they appear on the plane. I had to save two separate diffuse textures for the pebbles and the ground then combine them in Photoshop, but I was hoping there was a way to save this in Blender to avoid those extra steps.
[![enter image description here][5]][5]

[<img src="https://blend-exchange.com/embedImage.png?bid=aPNRZ9jP" />](https://blend-exchange.com/b/aPNRZ9jP/)


  [1]: https://i.sstatic.net/fwV43.jpg
  [2]: https://i.sstatic.net/KMiTG.jpg
  [3]: https://i.sstatic.net/JTsPq.jpg
  [4]: https://i.sstatic.net/RvHAp.jpg
  [5]: https://i.sstatic.net/kC0yd.jpg