14 C.F.R. § 27.1147
Mixture controls
If there are mixture controls, each engine must have a separate control and the controls must be arranged to allow—
(a) Separate control of each engine; and
(b) Simultaneous control of all engines.
If there are mixture controls, each engine must have a separate control and the controls must be arranged to allow—
(a) Separate control of each engine; and
(b) Simultaneous control of all engines.