(a) If by reason of sickness or other disability a judge before whom the defendant is being tried is unable to continue presiding over the trial without the necessity of a continuance, he may in his discretion order a mistrial.
(b) If by reason of absence, death, sickness, or other disability, the judge before whom the defendant is being or has been tried is unable to perform the duties required of him before entry of judgment, and has not ordered a mistrial, any other judge assigned to the court may perform those duties, but if the other judge is satisfied that he cannot perform those duties because he did not preside at an earlier stage of the proceedings or for any other reason, he must order a mistrial.