Hello Severin, 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.
The image has been created using: http://atom.smasher.org/error/?icon=Attention&style=xp&title=Can%27t+find+sys.dat&url=&text=Reload.+The+system+was+unable+to+allocate+the+data+in+the+ram.+It+will+no+purge+%26+reload+it.&b1=&b2=Ok&b2g=x&b3=
[Session] ARSession (0x7faae1d0d940): Unable to run the session, configuration is not supported on this device: <ARWorldTrackingConfiguration: 0x6000000d6b90 planeDetection=None worldAlignment=Gravity lightEstimation=Enabled detectionImages=> 2018-05-29 12:28:16.511617+0200 argianna[2065:40866] [Session] Session (0x7faae1d0d940): did fail with error: Error