The external pages in RadSplitter are rendered like a standard IFRAME and the correct way to reference them on the main page is the one demonstrated in this Knowledge Base article: Referencing content page in a splitter pane from the parent page and vice versa.
(especially the second part, which is about the RadSpliter for ASP.NET AJAX ).
Please, note that this solution is not applicable in every scenario - e.g. if both the parent page and the content page are not in the same security domain
the Telerik team
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