It’s definitely something we’re aware of. At the moment, it only makes sense to use multiple materials if your mesh has multiple submeshes (in which case you can use a different material per submesh). We would like to support Unity’s behavior of layering materials on the same MeshRenderer eventually, but for the moment, the only way to get that behavior is to use multiple overlapping objects (each with their own MeshFilter/MeshRenderer and a single material) along with a PolySpatial Sorting Group (probably with “Post Pass”, to avoid z fighting) to explicitly control the rendering order.
Yes: you can only use multiple materials if there are multiple submeshes. We still plan to add support for layering materials, though (because the only way to do that in RealityKit is to use sorting groups), it will not work with explicit sorting groups. Currently, the only way to layer materials is to use multiple GameObjects (each with their own MeshRenderer) and use explicit sorting groups to control their render order.