Can a defendant who is not represented by counsel participate in the conduct of the trial during the trial?

"We are not saying that the trial court may not in its discretion, upon what it may determine to be good cause shown, permit a party who is represented by counsel to participate in the conduct of the case ... or permit a defendant who appears in propria persona to employ an attorney to sit by him and advise him during the presentation of the case in court, or even appoint an attorney (with the latter's consent) to render such advisory services to an indigent defendant who wishes to represent himself (citation). These matters are within the sound discretion of the trial judge, who is in a position to appraise the courtroom situation and determine what procedure will best promote orderly, prompt and just disposition of the cause. The court, however, should not permit a litigant both to have counsel and to actively participate in the conduct of the case (as by conducting examination of witnesses, interposing objections, arguing points of law or of fact, addressing the jury, etc.) unless the court on a substantial showing determines that in the circumstances of the case the cause of justice will thereby be served and that the orderly and expeditious conduct of the court's business will not thereby be substantially hindered, hampered or delayed." (People v. Mattson, supra, 51 Cal.2d at p. 797, 336 P.2d 937.)