Hi
It is not possible to place more than one ScriptManager in a page. Why you want to put more than one on a page.
A Web page can contain only one http://forums.asp.net/mref/T_System_Web_UI_ScriptManager.aspx control, either directly on the page itself or indirectly inside a nested or parent component. The ScriptManagerProxy control enables you to add scripts and services to content pages and to user controls, where the master page or host page already contains a http://forums.asp.net/mref/T_System_Web_UI_ScriptManager.aspx control.
For more info refer to the below link
http://msdn.microsoft.com/en-us/magazine/cc163354.aspx