Nell'attesa della risposta di qualcuno, ecco cosa avevo fatto, e se ben intendo, è proprio quello che mi hai suggerito te:

codice:
SqlParameter pUsername = new SqlParameter();
                pUsername.ParameterName = "@Username";
                pUsername.DbType = DbType.String;
                pUsername.Direction = ParameterDirection.Input;
                pUsername.Value = txtUsername.Text;


                SqlParameter pEmail = new SqlParameter();
                pEmail.ParameterName = "@Email";
                pEmail.DbType = DbType.String;
                pEmail.Direction = ParameterDirection.Input;
                pEmail.Value = txtEmail.Text;


                using (SqlConnection conn = new SqlConnection(builder.ConnectionString))
                {
                    try
                    {
                        conn.Open();
                    }


                    catch (Exception excMessage)
                    {
                        Response.Write(excMessage.ToString());
                    }


                    using (SqlCommand cmqQuery = new SqlCommand("SELECT COUNT(*) FROM Tabella_Account WHERE Username=@Username OR Email=@Email", conn))
                    {
                        cmqQuery.Parameters.Add(pUsername);
                        cmqQuery.Parameters.Add(pEmail);
                        int count = (int)cmqQuery.ExecuteScalar();
                        lblUsername.Text = count.ToString();
                   //     lblUsername.Text = MailStatus;


                    }
                }
Cosa ne pensi adesso?