Venkat,
There are several ways to do this. Are you using web.sitemap or some other means of describing your menu items? Here's an example with web.sitemap, if you're using static nodes, you would need to write code to set the root nodes.
web.sitemap:
<?xml version="1.0" encoding="UTF-8"?>
<siteMap>
<siteMapNode url="default.aspx" title="root">
<siteMapNode url="subMenu.aspx?A=1" title="Node One">
<siteMapNode url="a.aspx" title="One.One"></siteMapNode>
<siteMapNode url="b.aspx" title="One.Two"></siteMapNode>
<siteMapNode url="c.aspx" title="One.Three"></siteMapNode>
<siteMapNode url="d.aspx" title="One.Four"></siteMapNode>
</siteMapNode>
<siteMapNode url="subMenu.aspx?B=1" title="Node Two">
<siteMapNode url="e.aspx" title="Two.One"></siteMapNode>
<siteMapNode url="f.aspx" title="Two.Two"></siteMapNode>
<siteMapNode url="g.aspx" title="Two.Three"></siteMapNode>
<siteMapNode url="h.aspx" title="Two.Four"></siteMapNode>
</siteMapNode>
</siteMapNode>
</siteMap>
topmenu.aspx
<%@ Page Language="VB" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:SiteMapDataSource ID="SiteMapDataSource1" runat="server" ShowStartingNode="false" />
<asp:Menu ID="Menu1" runat="server" DataSourceID="SiteMapDataSource1" Target="sub" MaximumDynamicDisplayLevels=0>
</asp:Menu>
</div>
</form>
</body>
</html>
submenu.aspx
<%@ Page Language="VB" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:SiteMapDataSource ID="SiteMapDataSource1" runat="server" StartFromCurrentNode="true" ShowStartingNode="false" />
<asp:Menu ID="Menu1" runat="server" DataSourceID="SiteMapDataSource1" Target="main">
</asp:Menu>
</div>
</form>
</body>
</html>
default.html
<frameset rows="100%" cols="50%,50%">
<frame name="top" src="topMenu.aspx">
</frame>
<frame name="sub" src="subMenu.aspx?A=1"></frame>
</frameset>
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