When you use the RadPane's ContentUrl
property you actually create an IFRAME element and place it in the RadPane. The described order of loading is the expected one, when using IFRAMEs and you can test this by replacing the RadSplitter's structure with two IFRAMEs.
What I can suggest is to hook up the RadSplitter's OnClientLoaded
handler and implement a proper client-side handler. In this handler you should reference the RadPanes by using the $find
method and then set their ContentUrl in the desired order by using the RadPane's set_contentUrl method.
the Telerik team