Linkbutton Is Not Doing Postback

I have 2 link button in my page and every thing was working fine until i added window.onload() method on my page. After adding window.onload() first link button is working but not the second button. I am using update panel in my page

<script>
function PageTest() { //do some task}
window.onload = PageTest;
</script>
<asp:UpdatePanel ID="UpdPanel" ChildrenAsTriggers="true" UpdateMode="Conditional" runat="server">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="linkbutton1" EventName="Click" />
</Triggers>
<ContentTemplate>
//some code
<asp:linkbutton id="linkbutton1" runat="server" cssclass="btn" onclick="linkbutton1_Click" OnClientClick ="return Ischecked('abc');">
</ContentTemplate>
</asp:UpdatePanel>
<asp:UpdatePanel ID="UpdatePanel1" ChildrenAsTriggers="true" UpdateMode="Conditional" runat="server">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="linkbutton1" EventName="Click" />
</Triggers>
<ContentTemplate>
<asp:placeholder id="placeholder" visible="false" runat="server">
<asp:linkbutton id="linkbutton2" runat="server" cssclass="btn" onclick="linkbutton2_Click" OnClientClick="return IsChkBoxchecked('xyz')" >
<span>Submit</span>
</asp:linkbutton>
</asp:placeholder>
</ContentTemplate>
</asp:UpdatePanel>