Hello Johannes, thank you for posting on the Forum. In order to assign two different materials in SimScale, each one of the solid volumes should have their own faces which will be a contact in the simulation.
It’s not currently possible to have two solids share one interface, and assign them two different materials. That’s why such operation, splitting a solid volume so that two different volumes can share only one face, is not available in the CAD mode.
Hope this helps.