First I want to draw attention on our RadChartView components. The RadChartView is our new charting suit which was designed to replace the old RadChart. It has better implementation and performance, and also is very flexible and easy to set up. You can read about the advantages of the new components in the RadChartView vs. RadChart.
As for your requirement I am not completely sure that I can understand it from the provided picture. However, my guess is that you want when you click on a series from your first chart, this series to be added in the second chart. Can you confirm this is your case? If so, you can take a look at the ChartSelectionBehavior which is part of the RadCartesianChart. Basically you can subscribe for the SelectionChanged event of the behavior and then in the event handler add the series in the other chart.
Note that the SelectionChanged event will be fired only if you click on a DataPoint. If you click on the line between two points the event wont be invoked.
In addition I prepared a sample project.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos and start improving your app based on facts, not hunches.