Hi! Hope this solves your problem!!! i've checked this in my local env. it's working as you expected..
<asp:Menu ID="Menu1" runat="server" orientation="Horizontal" StaticDisplayLevels="2" datasourceid="SiteMapDataSource1">
</asp:Menu>
<asp:SiteMapDataSource ID="SiteMapDataSource1" runat="server" />
and in sitemap file
="1.0"="utf-8"
<siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >
<siteMapNode url="Home" description="Home">
<siteMapNode title="Home" description="Home" url="~/default.aspx" />
<siteMapNode title="Products" description="">
<siteMapNode title="mobile" description="Our products" url="~/mobile.aspx">
<siteMapNode title="web based" url="~/webbased.aspx" />
<siteMapNode title="windows" url="~/windows.aspx" />
</siteMapNode>
</siteMapNode>
</siteMapNode>
</siteMap>
Ref:
http://msdn.microsoft.com/en-us/library/ecs0x9w5(v=vs.80).aspx[
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Mark it if you find this useful!!
~~SATZ~~