Hello,
I have a similiar problem but im using vanilla asp.net 4 (no sitefinity).
I have the radajaxmanager inside the masterpage but im using the radajaxmanagerproxy on a page to point to which controls should be updated through ajax, som of theese controls are custom user controls with their own OnChecked OnUpdateD events.
I have tried the proxy and your example from code-behind, doing it from the code without success. It only fires the first loadingpanel but when I make the second ajax request it doesnt open the loadingpanel but the ajax request works fine.