Texbox2 Set To Requiredfieldvalidator1.controltovalidate Instead Of ValidationTB Textbox?

I have site with two text boxes and dynamically create validation control. This is code from .aspx file:

<form runat="server">
<asp:TextBox AutoPostBack="true" ID="TextBox1" Text="" runat="server" Width="200px"
OnTextChanged="TextBox1_TextChanged"></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server" Visible="True" Width="200px"AutoPostBack="true"></asp:TextBox>
<asp:Panel ID="Panel1" runat="server">
</asp:Panel>
<asp:TextBox ID="ValidationTB" runat="server" Visible="true"></asp:TextBox>
</form>
This is my code-behind:
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (TextBox2.Visible)
{
if (!String.IsNullOrEmpty(TextBox1.Text) && String.IsNullOrEmpty(TextBox2.Text))
{
RequiredFieldValidator RequiredFieldValidator1 = new RequiredFieldValidator();
RequiredFieldValidator1.Enabled = true;
RequiredFieldValidator1.ErrorMessage = "Second field required";
RequiredFieldValidator1.Display = ValidatorDisplay.Dynamic;
RequiredFieldValidator1.ControlToValidate = "TextBox2";
Panel1.Controls.Add(RequiredFieldValidator1);
RequiredFieldValidator1.Validate();
}
if (!String.IsNullOrEmpty(TextBox2.Text) && String.IsNullOrEmpty(TextBox1.Text))
{
RequiredFieldValidator RequiredFieldValidator1 = new RequiredFieldValidator();
RequiredFieldValidator1.Enabled = true;
RequiredFieldValidator1.ErrorMessage = "First field required";
RequiredFieldValidator1.Display = ValidatorDisplay.Dynamic;
RequiredFieldValidator1.ControlToValidate = "TextBox1";
Panel1.Controls.Add(RequiredFieldValidator1);
RequiredFieldValidator1.Validate();
}
if (!String.IsNullOrEmpty(TextBox2.Text) && !String.IsNullOrEmpty(TextBox1.Text))
{
if (Convert.ToDateTime(TextBox2.Text) < Convert.ToDateTime(TextBox1.Text))
{
ValidationTB.Text = null;
RequiredFieldValidator RequiredFieldValidator1 = new RequiredFieldValidator();
RequiredFieldValidator1.Enabled = true;
RequiredFieldValidator1.ErrorMessage = "Bad range of dates";
RequiredFieldValidator1.Display = ValidatorDisplay.Dynamic;
RequiredFieldValidator1.ControlToValidate = "ValidationTB";
Panel1.Controls.Add(RequiredFieldValidator1);
RequiredFieldValidator1.Validate();
}
}
}
}
protected void TextBox1_TextChanged(object sender, EventArgs e)
{
RegularExpressionValidator RegularExpressionValidator1 = new RegularExpressionValidator();
RegularExpressionValidator1.ValidationExpression = @"^[0-9]{4}-(((0[13578]|(10|12))-(0[1-9]|[1-2][0-9]|3[0-1]))|(02-(0[1-9]|[1-2][0-9]))|((0[469]|11)-(0[1-9]|[1-2][0-9]|30)))$";
RegularExpressionValidator1.Enabled = true;
RegularExpressionValidator1.ErrorMessage = "Bad format of date";
RegularExpressionValidator1.Display = ValidatorDisplay.Dynamic;
if (!String.IsNullOrEmpty(TextBox1.Text))
{
RegularExpressionValidator1.ControlToValidate = "TextBox1";
Panel1.Controls.Add(RegularExpressionValidator1);
RegularExpressionValidator1.Validate();
}
if (!String.IsNullOrEmpty(TextBox2.Text))
{
RegularExpressionValidator1.ControlToValidate = "TextBox2";
Panel1.Controls.Add(RegularExpressionValidator1);
RegularExpressionValidator1.Validate();
}
}
}

TextBox ValidationTB is just to make validate on empty control. This validation doesn't work, when I try:

1. To first textbox enter for example: 2009-09-09
2. To second textbox enter for example: 2009-10-09 Now, everything is OK.
3. I change my first textbox on for example 2009-12-09 I get error Bad range of dates - it's OK.
4. I correct first textbox on 2009-09-09, message disappear-OK.
5. Again enter to first textbox 2009-12-09 - I don't have error, but it should be.

What strange - in debug mode I can see, that in code:

if (Convert.ToDateTime(TextBox2.Text) < Convert.ToDateTime(TextBox1.Text))
{
ValidationTB.Text = null;
RequiredFieldValidator RequiredFieldValidator1 = new RequiredFieldValidator();
RequiredFieldValidator1.Enabled = true;
RequiredFieldValidator1.ErrorMessage = "Bad range of dates";
RequiredFieldValidator1.Display = ValidatorDisplay.Dynamic;
RequiredFieldValidator1.ControlToValidate = "ValidationTB";
Panel1.Controls.Add(RequiredFieldValidator1);
//In debug window: RequiredFieldValidator1.ControlToValidate = "TextBox2"
RequiredFieldValidator1.Validate();
}

instead of ValidationTB control, RequiredFieldValidator1.ControlToValidate is set to TextBox2 (it isn't empty, so I haven't error message). Why TextBox2 is set to RequiredFieldValidator1.ControlToValidate instead of ValidationTB textbox and how I could solve this?