I would suggest you to use jquery, in order to achieve the desired functionality. I have prepare a sample page for you, demonstrating a possible implementation. In addition, here is a video
demonstrating the behavior at my end. Please correct me, if I had misunderstood the scenario you attempt to achieve.
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