I obtained a hexpress surface to volume mesh of the countra-rotating propeller (I followed the meshing tutoral "Meshing the PPTC Propeller using surface-to-volume meshing"). Should I add a rotating sub-domain or is it possible to make the individual props rotate withou it?
my advice would be to create 3 separate meshes: 1 for the external part and 1 per propeller unit. Making the propeller unit with S2V (surface to volume) is totally fine and you can make the external one with V2S (Volume to surface). This means the external one should have 2 cylindrical holes so that you can combine all meshes together later on. See picture attached.
To combine all meshes in Fine Marine: