Mo. Rev. Stat. § 433.100: Failure to prosecute such action exonerates surviving surety.

If such action be not begun by the party having the right thereof, within a reasonable time after being notified as above provided so to do, and proceeded in with due diligence in the ordinary course of law to a final determination of the issues, the surety giving such notice shall be exonerated from liability to the person so notified; provided, that two or more sureties may join in the same notice; provided further, that the service of notice on the administrator or executor of the estate, or the waiver thereof, shall be deemed to be the commencement of such action within the meaning of sections 433.090 and 433.100.