问个updatepanel和验证控件配合的问题

前台：
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %><!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></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
 <asp:UpdatePanel ID="UpdatePanel2" UpdateMode="Conditional" runat="server">
        <ContentTemplate>
                  <asp:TextBox ID="TextBox2" runat="server" Height="29px"></asp:TextBox>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
            </Triggers>
    </asp:UpdatePanel>
        <br />
        <br />
        <asp:UpdatePanel UpdateMode="Conditional" runat="server">
        <ContentTemplate>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:CustomValidator ID="CustomValidator1" runat="server" 
            ControlToValidate="TextBox1" ErrorMessage="dddd" Text="aaaaa" 
                onservervalidate="CustomValidator1_ServerValidate" ValidateEmptyText="True"></asp:CustomValidator>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="Button2" EventName="Click" />
            </Triggers>
    </asp:UpdatePanel>
        <asp:Button ID="Button2" runat="server" onclick="Button2_Click" Text="Button" />
    </div>
    </form>
</body>
</html>
后台using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        try
        {
            Session["a"] = "aa";
            TextBox2.Text = "sda";
            //Response.Redirect("Default2.aspx", true);
            int a = 2;
        }
        
        catch (Exception)
        {            Session["a"] = "bb";
        }
        
        
    }
    protected void Button2_Click(object sender, EventArgs e)
    {    }
    protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
        
        if (TextBox1.Text == "")
            args.IsValid = false;
        else
            args.IsValid = true;
    }
}
问题，当点击按钮2的时候，验证触发，红字显示，但当点击按钮1时候，红字消失，我如何能让红字保持呢？要求就是2个按钮各管各的updatepanel 互不相干