ok, I am attaching a scene
you have 2 sets of objects in it:
-set 1 - only materials used, there are two materials you can apply:
mat 1 - light mtl, it has no shading (no reflections, no bump, no shadows cast or received)
mat 2 - corona mtl, casts shadows, you may use diffuse, reflectivity, bump etc with it
-set 2 - it is duplicated geometry with "flipped normals" faked in Corona:
it has mat3 applied - pure black corona mtl with front/back map in opacity slot
I also set secondary solver to none and max ray depth to 1 to make everything render ultra fast.
Have fun!