title: SceneKit using 2 materials with different Texture Coordinates on same geometry
body: <p>I try to apply two different materials on a same geometry. 
  One material for <strong>tiling</strong> and second for <strong>Ambient Occlusion</strong> and other effects.
  <img src="https://help.autodesk.com/cloudhelp/2016/ENU/Maya/images/GUID-240372D2-D07F-42B3-836D-4736EB4223B9.png" alt="example"></p>
  
  <p>I export a model with 2 <strong>UVSet</strong> (it's OK) in <strong>DAE</strong> format. When I try to apply the second material, it never appear. On second material, I try to apply blendMode, transparencyMode, png file, etc. Nothing works :((
  If I change mappingChannel on the first material, mapping is OK !</p>
  
  <pre><code>let mat1 = SCNMaterial()
  mat1.name = "Mat1"
  mat1.diffuse.mappingChannel = 0
  mat1.diffuse.contents = UIImage(named: "somePNGFile.png")
  
  
  let mat2 = SCNMaterial()
  mat2.name = "Mat2"
  mat2.diffuse.mappingChannel = 1
  mat2.diffuse.contents = UIImage(named: "otherPNGFile.png")
  
  cube.geometry!.materials = [mat1, mat2]
  </code></pre>
  
  <p>Here is my <strong>SCN file</strong> : <a href="https://drive.google.com/open?id=1ddbE815hlS0VbKPsbwcJoS9cL8hOf6Mt" rel="nofollow noreferrer">https://drive.google.com/open?id=1ddbE815hlS0VbKPsbwcJoS9cL8hOf6Mt</a></p>
  
  <p>What's wrong in my code ? Maybe the 3D File... 
  If you have an idea, a suggestion, a solution, it's great.</p>
  
  <p>Thanks</p>