I would suggest using a custom DragDropBehavior for the destination RadListBox control:
- Inherit the ListBoxDragDropBehavior
- Override the Drop method. The DragDropState object gives you information about the destination itemssource, draggeditems, insert index and etc. Here you could add some custom logic.
- Set the CustomDragDropBehavior to the destination RadListBox.
You can find the attached sample as well. Hope this helps.
the Telerik team