frmMasterPage.Master:
==================
<form id="form1" runat="server">

<asp:ScriptManager ID="scriptManager1" runat="server">

</asp:ScriptManager>
<script type="text/javascript" language="javascript">

function ShowContentTwo() //while set as start page is frmContentTwo.aspx
{

alert("from two content");
this.id="frmMasterPage";
alert(this.id);
var elemId=document.getElementById('<%=form1.FindControl("ContentPlaceHolder2").ClientID%>');
//var elemId=document.aspnetForm.frmMasterPage_ContentPlaceHolder2;
alert(elemId);

elemId.style.display='block';
elemId.style.visibility='visible';

alert("from master two");

call()

}

</script>

<div>
<asp:UpdatePanel runat="server">
<ContentTemplate>
<a id="anchorLinkOne" runat="server" href="javascript:void(0)" onclick="ShowContentOne1()" style="color: blue; text-decoration: underline;cursorointer">Div One</a>
</ContentTemplate>
</asp:UpdatePanel>
<br />
<asp:UpdatePanel runat ="server" >
<ContentTemplate>
<a id="anchorLinkTwo" runat="server" href="javascript:void(0)" onclick="ShowContentTwo()" style="color: blue; text-decoration: underline;cursorointer">Div Two</a>
</ContentTemplate>
</asp:UpdatePanel>
<br />

</div>
<div>
<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
</asp:contentplaceholder>
<asp:ContentPlaceHolder ID ="ContentPlaceHolder2" runat="server">
</asp:ContentPlaceHolder>
</div>
</form>

frmContentPageOne.aspx :
===================
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<div id="divContentOne" runat="server" style="display:none;background-color:Red;height:500px;width:500px">

</div>

</asp:Content>

frmContentTwo.aspx :
=================
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">

<div id="divContentTwo" style="display:none;background-color:Green;height:500px;width:500px">

</div>
</asp:Content>