The reason for the experienced behavior is the following. When you use the ContentUrl property of the RadPageView its content is rendered in an iframe. When this pageview is shown for the first time a "Get" request is made in order the content of the page which is referenced by url to be loaded. In your case you are trying to manipulate a content which is not yet loaded since you haven't selected the second tab in order its pageview to make a request for its content. If you want the content of all pageviews to be loaded at once you should use user controls, not referencing a page with the ContentUrl property.
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