I can see the correct material is displayed when I cull the backface of each ShaderMaterial:
Is there a difference in rendering between FreeCamera and ArcRotateCamera?
Can anyone see why/where the shaderMaterials are overwriting each other?
Is there a better way to explain what is going on?
You can better grasp what is happening in this pg https://playground.babylonjs.com/#LRF4JU#5 where the clear color of the rtt have been changed and then playing in the inspector
It is because you see the alternate portal through the render targets. You can see here the correct behavior without renderlist https://playground.babylonjs.com/#LRF4JU#6