Mo. Rev. Stat. § 484.170

Where two or more licensed attorneys at law of this state are associated together as copartners in the practice as attorneys or counselors at law, and one of such attorneys shall be the prosecuting or assistant prosecuting attorney, or circuit or assistant circuit attorney, it shall be unlawful for such other partner or partners to appear and defend in any of the courts of this state any person charged with a misdemeanor or felony, when by law it shall be the duty of such prosecuting or circuit attorney to appear and prosecute, and this section shall apply even though such partnership may exist only for the transaction of civil business.