To display both text values in a listbox item, you could use the Text and Description properties of RadListBoxItem. I'm not sure what exactly is the case, but if the values are not textual, you may also create your own descendant of RadListBoxItem which adds additional properties to suit your needs. Alternatively, you can use the Tag property to store any value associated with a particular item.
Nevertheless, although the RadListBox.Items collection can accept instances of RadItem, direct instantiation of this class is not enough to show anything in the RadListBox. This is due to the architecture of our controls, and the role of the RadItem as an abstract class (please read more about this in our documentation: http://www.telerik.com/help/winforms/architecture_elementsoverview.html ).
If you have any additional questions, please don't hesitate to write.