I am not sure whether I understand the scenario properly. Is the external page rendered as an external content for any of the RadPanes of the Splitter or is it placed inside a standard content page for the master one? In the primer case, you can find detailed information on how to manage the data between the page containing the Splitter and the external page here: Referencing the external content of a pane
As for the second case - you can use RadAjaxManager and follow the approach described in this help article: MasterPages: Ajaxify and Update Controls in Master and Content Page
If on of the above is the case, though, please explain in more details what you are trying to achieve so I can hep you further. Preparing a sample runnable page demonstrating it will be of great help understanding the scenario at hand.
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