ASP.NET - Activar botones al iniciar sesion

<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="a(); return false;"/>

<asp:Button ID="Button2" runat="server" Text="Button" OnClientClick="b(); return false;" />

<script type="text/javascript">

   function a() {

       alert('1');

       document.getElementById('<%=Button1.ClientID %>').disabled = true;

       document.getElementById('<%=Button2.ClientID %>').disabled = false;

      }

   function b() {

       alert('2');

       document.getElementById('<%=Button2.ClientID %>').disabled = true;

       document.getElementById('<%=Button1.ClientID %>').disabled = false;

      }

 </script>

private void textBox1_TextChanged(object sender, EventArgs e)

{

	if ((textBox1.Text != "") && (textBox2.Text != "")) button1.Enabled = true;

	else button1.Enabled = false;

}

private void textBox2_TextChanged(object sender, EventArgs e)

{

	if ((textBox1.Text != "") && (textBox2.Text != "")) button1.Enabled = true;

	else button1.Enabled = false;

}

<form action="" method="post" name="frm" id="frm">

  <p>

    <select name="select">

      <option value="0" selected>Predeterminado</option>

      <option value="1">Uno</option>

      <option value="2">Dos</option>

    </select>

</p>

  <p>

    <input type="submit" name="Submit" value="Enviar">

</p>

</form>