Text in force at 23-63: from 23-43, effective 10 May 1993.
If there are mixture controls, each engine must have a separate control, and each mixture control must have guards or must be shaped or arranged to prevent confusion by fuel with other controls. (a)[(1) The controls must be grouped and arranged to allow-- (i) Separate control of each engine; and (ii) Simultaneous control of all engines. (2) The control must require a separate and distinct operation to move the control toward lean or shut-off position. (b) For reciprocating single-engine airplanes, each manual engine mixture control must be designed so that, if the control separates at the engine fuel metering device, the airplane is capable of continued safe flight and landing.]
58 FR 18974 · Source