This feature is available and it is used by setting the Stacked property of the first BarSeries in your chart to true. You can see it used in this online demo
(although the series type there is ColumnSeries). It is also available in the configurator (Quarter 1 Settings) in this online demo
, yet setting only this property will break the display of the chart because this demo has a lot more properties hardcoded for the sake of demonstration and thus the configured axis range will not be able to accommodate the combined values.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed