<div>
    Main menu:
    <asp:Menu ID="menu1" runat="server" Orientation="Horizontal">
        <Items>
            <asp:MenuItem Text="A">
            <asp:MenuItem Text="B">
        </Items>
    </asp:Menu>
</div>
<div>
    Sub menu:
    <asp:Menu ID="menu2" runat="server" Orientation="Horizontal">
    </asp:Menu>
</div>
<div>
    Selected sub menu item:
    <asp:Label ID="lbl1" runat="server">
</div>

protected void Page_Init(object sender, EventArgs e)
{
    menu1.MenuItemClick +=
        new MenuEventHandler(menu1_MenuItemClick);
    menu2.MenuItemClick += new MenuEventHandler(menu2_MenuItemClick);
}

void menu1_MenuItemClick(object sender, MenuEventArgs e)
{
    menu2.Items.Clear();
    menu2.Items.Add(new MenuItem(e.Item.Text + "-1"));
    menu2.Items.Add(new MenuItem(e.Item.Text + "-2"));
}

void menu2_MenuItemClick(object sender, MenuEventArgs e)
{
    lbl1.Text = e.Item.Text;
}