Yes, you can try a value converter that will take the class with the two collections and return the union of them. This will sort out the hierarchy but then you may still need two templates. In this case, you can use the ItemTemplateSelector and return a different template based on some criteria (Type I imagine).
A slightly better approach IMO will be to add a property to your class that returns the union of the two collections. This can be done easily with linq, you can return
If you are using a generated class (like from a WebService) you will find that these classes are generated as partial so that you can easily extend them for scenarios like this one.
Unfortunately both these approaches will not deal with the case when the two collections are observable and you need to be notifies when objects are added/removed. If this is the case for you, I can reply with more details how this can be implemented.
the Telerik team