At present our RadChart for ASP.NET AJAX does not expose a property or a way out-of-the-box to add a secondary X Axis. We are sorry for the inconvenience.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!