The code behind that you pasted is the same as in our Multiple Resource Values. If you want to use multiple resources in an Advanced Template we recommend using code as in this demo. We usually recommend implementation with a user control with multiple check boxes as in the example. If you want to replace it with RadCombobox you will have to add it in the MultipleValueResourceControl page and populate each checked resource.
Hope this information will be helpful.
All the best,
the Telerik team
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