Unfortunately this is not possible at the moment. We could try to workaround it but some of the built-in functionalities of the RadDiagram won't work correctly because of it.
One way to do this will be with custom RadDiagram and custom style that uses two ItemHosts (DiagramSurfaces) for the different shapes.
I've attached a sample project demonstrating this approach but as I said - not everything (e.g. ManipulationAdorner and Connections) is working as expected with the special NoZoom shapes so could you please examine it and see if this is what you're looking for.
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