Mo. Rev. Stat. § 477.020
The decision of the majority of the judges of the supreme court or of any district of the court of appeals shall be the decision of the court, but if in any case the judges shall be equally divided in opinion, the parties to the cause may agree upon some person learned in the law, who shall act as special judge in the cause, and who shall sit therein with the court, and give decision in the same manner and with the same effect as one of the judges; and such agreement shall be in writing, signed by the parties or their attorneys of record, and filed with the papers and form a part of the record in the cause. If the parties cannot agree upon a special judge, the court shall appoint, by an order of record, some person possessing the qualifications aforesaid, to act as such special judge.
§ 477.020, RSMo