RadComboBox does not support this out of the box. You should modify the control template to add headings for the columns in the control. This is not very flexible, because you will have to provide different control template (with different headings) for each combo box instance that has different columns in its drop down. We are aware that multi-column combo box was very often requested feature of RadComboBox for ASP.NET and it is on our list for future development. However, at this I am unable to provide a specific date when we will have it.
the Telerik team
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