Mo. Rev. Stat. § 534.290: New tenant may maintain action, when.

Where the action of unlawful detainer is brought by a landlord against his tenant holding over after the termination of the tenancy, it shall be no defense to show that the plaintiff, before such termination, has let the premises to another person; and in case of such new letting, the new tenant after demand made in writing requiring the delivery of possession of such premises and setting forth his title, if the possession be refused, may maintain an action of unlawful detainer against the tenant holding over, if there be no such action by the landlord pending.