Is this not possible because only one view can access the camera per time?
No, the issue is related to the underlying rendering pipeline. Multiple views can display the camera image at the same time, for example, two ARSCNViews can dispaly the camera image simultaneously.
Again, you should be able to combine 3D and 2D elements using soley an ARSCNView.
You cannot use an ARSCNView and an ARSKView simultaneously in the way that you are attempting to do so (i.e. you cannot have an ARSKView on top of an ARSCNView or vice versa).
This is the response I got after contacting the apple support because I’ve wanted to combine a ARSCN and a ARSKView in the Same view Controller. I am contacting you because I have a problem with my iOS app. I love ARKIT, but unfortunately I have a little issue with my app. My app uses both SceneKit and SpriteKit (ARSCNView and a ARSKView). Both parts separately are working 100% fine, but as soon as I combine both technologies in one ViewController, one of the technologies stops working. I get an exception saying that a sensor has failed. There is no pattern to wich Technologie is working and wich not. Sometimes the ARSCNView works and then the ARSKView doesn’t load, or it’s the other way around and the ARSKView works while the ARSCNView doesn’t. When the ARSCNView doesn’t work, it doesn’t only display nothing, but the Frame Rate stays at zero. Can I send you my project? In this E-Mail you will find a video attachment showing the issue. First the SpriteKit part worked and then the SceneKit part.