You have correctly noticed that when you want to update the content of a RadPane with AJAX you should wrap it in a panel and update the panel. However, this is teh case when you have internal content in teh pane. When you specify a ContentUrl, the RadPane creates an IFRAME element in which to load the external page in it and that is why the panel solution will not work. If you want to keep your setup, I suggest to remove the panel and to try updating the whole splitter.
On a side note, I recommend to use the RadPane's client-side API to get the desired result - all you should do is to reference the particular RadPane and to pass the desired url to its set_contentUrl
method which is listed below:
the Telerik team