Well in this case you would have to do everything dynamically in code behind. Basically make a check if RadAjaxManager already exists and if this is the case do not add a second one. Here is a sample check:
public static RadAjaxManager GetCurrent(Page page)
if (page == null)
throw new ArgumentNullException("page");
return (page.Items[typeof(RadAjaxManager)] as RadAjaxManager);
Hope this helps.
the Telerik team