(1) Federal. The rules of practice and procedure of the federal agency govern in any proceeding in which the commission participates jointly with a federal agency.

(2) State. The rules of the state in which the hearing is held govern in any proceeding in which the commission participates jointly with the administrative body of another state or states, unless otherwise agreed by the participating agencies.

(3) Who may appear. Any person entitled to appear in a representative capacity before any of the agencies involved in a joint hearing may appear in the joint hearing.