Note that using UpdatePanel and RadAjaxManager in one application for updating same part of the page is not supported scenario. In your application I would suggest you to use single AjaxManager on the main page for ajaxify the controls in the subpages by getting reference to this manager and add its setting dynamically. Please review the following help topics:
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.