Serrin's advice is exactly correct. You should use the RadAjaxManager to enable Ajax to update multiple areas on a page when a single event occurs. You can configure RadAjaxManager to handle all of your Ajax interactions- simply define controls that initiate events (via AjaxSetting) and then the controls that should be updated on the page after that event occurs (AjaxUpdatedControl).
If you're working with MasterPages or UserControls, you can place a single RadAjaxManager on your MasterPage and then use the RadAjaxManagerProxy to configure Ajax settings on your ContentPages and UserControls.
Hope that helps.