One possible solution is to set CustomAttribute (for example you could use the ID of the item) for every item in the RadTreeView control. Having the ID of the currently selected item you could easily get the corresponding data for the first RadComboBox's selected item, and populate the second RadComboBox with it.
To see how custom attributes are made for the RadTreeView, take a look at the following online demo:
RadTreeView: Custom Attributes
and/or the online help article: RadTreeView: Custom Attributes Overview
For more information about the work of the related combos, you could refer to the following demo at our website: RadComboBox: Related Combos
the Telerik team
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