I am not completely sure about your exact scenario and how you update the content - though an ajax request or by client-side code but I believe that the following KB articles will be helpful:
1) Use Ajax request to update part of an external page, loaded in a RadPane - this article has another scenario than yours but the logic can be slightly modified to get what you need by using ajax.
2)Referencing content page in a splitter pane from the parent page and vice versa - this is how you can access the parent and the external page to manipulate them:
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