How do I handle multiple sql data reader in one form

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C#

Windows

form

I want to bind data with 2 different combo box for that i had use sql data reader twice time for bind data from 2 different table but it shows an error

SqlConnection conn = new SqlConnection("Data Source=HP\\SASI;Initial Catalog = test;Integrated Security=SSPI;");
        conn.Open();
        SqlCommand cmd = new SqlCommand();
        string query = "Select fn+''+ln from user_details where category='Doctor'";// position column from position table
        cmd.Connection = conn;
        cmd.CommandText = query;

        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            string myItem = dr[0].ToString();
            comboBox1.Items.Add(myItem);
        }
        conn.Close();

      SqlConnection conn1 = new SqlConnection("Data Source=HP\\SASI;Initial Catalog = test;Integrated Security=SSPI;");
        conn1.Open();
        SqlCommand cmd1 = new SqlCommand("select firstname+''+lastname from patient_table",conn1);
        SqlDataReader dr1 = cmd1.ExecuteReader();
        while (dr1.Read())
        {
            string name = dr[0].ToString();
            comboBox1.Items.Add(name);
        }