Unfortunately there is no event that fires when the ContentUrl value changes.
For further information on the client-side events, please, refer to the client-side events API documentation for RadSplitter
On a side note, in case you need those events to display a loading sign while a content page is loading, I am glad to inform you that there will be a property which allows this out of the box in the upcoming Beta release. Until the property is available you can check the following KB article:
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!