how we can have ToolkitScriptManager & ScriptManager both in master page

before i was using ScriptManager in the master page. and when i was running main page i had not any problem.

now i have decided to add combo box to main page , so i should add ToolkitScriptManager to the master page.

because of error , i removed ScriptManager of the master page but now my main page has completely post back and slow, how i can have both of them in the master page without error.

<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><asp:DropDownList ID="ddlProjectDocument" runat="server" Height="16px" AppendDataBoundItems="True"
         Width="212px" onselectedindexchanged="ddlProjectDocument_SelectedIndexChanged"
        AutoPostBack="True" onload="ddlProjectDocument_Load"
        style="font-family: 'Courier New', Courier, monospace" ><asp:ListItem Selected="True" Value="0">(select a value)</asp:ListItem></asp:DropDownList><br /><ajaxtoolkit:ComboBox ID="ComboBox1" runat="server" onload="ComboBox1_Load"
         AutoCompleteMode="Suggest"
         style="font-family: 'Courier New', Courier, monospace" ></ajaxtoolkit:ComboBox></ContentTemplate><Triggers><asp:PostBackTrigger ControlID="ddlProjectDocument"  /></Triggers></asp:UpdatePanel>