Text in force at 23-63: from 23-0, effective 1 Feb 1965.
(a) If there are propeller speed or pitch controls, they must be grouped and arranged to allow--
(1) Separate control of each propeller; and
(2) Simultaneous control of all propellers.
(b) The controls must allow ready synchronization of all propellers on multiengine airplanes.
29 FR 17955 · Source