Multiparty arbitration with co-parties having distinct interests on the same side. Silence in the arbitration center's rules. Gap-filling by the President of that institution, who gave precedence to the appointment of an arbitrator by only one of the co-parties. Failure to observe the principles of equal treatment and impartiality, which vitiated the constitution of the arbitral tribunal. No preclusion barring the prejudiced party from seeking annulment of the arbitration award (Article 19, § 2º, of the Brazilian Arbitration Act). Arbitration award annulled: