The functionality you need can be achieved by setting the TreeView to expand nodes on single click. Here's an article explaining how to do this: http://www.telerik.com/support/kb/article/b454K-eak-b454T-bgk.aspx
However, if you have only a 2-level hierarchy, I suggest you use the Site PanelBar control instead of Site Treeview. The PanelBar supports this functionality out of the box. You should just set the PersistStateInCookie
property to true
Hope this helps.
the Telerik team