You can write trigger between updatepanel1 and updatepanel2 as:
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger controlid="UpdateButton2" eventname="Click" />
</Triggers>