protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {              
>>>         SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString1"].ConnectionString);
            SqlCommand cmd = new SqlCommand("select id, [Name] from some_table", con);
            con.Open();
............ и т.д.

Object reference not set to an instance of an object. 

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
	<ConnectionStrings>
		<add name="ConnectionString1" connectionString="Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=Database1;Data Source=(local)"/>
		<add name="ConnectionString2" connectionString="Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=Database2;Data Source=(local)"/>
	</ConnectionStrings>