I found a workaround. First, I copied the table that defines the post code shapes into the same database as the hospital locations. Then I was able to create a double axis map with both shapes and locations.
The next problem that arose was, that once I wanted to filter the hospitals, the corresponding plz shapes would be removed.
I found a workaround for that, too, by duplicating the post code values and using them to create the shapes. This makes them independent from the hospital locations and they can now be filtered separately.
However, I don't think that's a very elegant solution and it uses more ressources, too. Is there a better way to do this? I included a workbook with some exemplary data, I hope it helps.
Shapes_Locations.twbx 12.9 MB