14 CFR § 25.1147

If there are mixture controls, each engine must have a separate control. The controls must be grouped and arranged to allow—

Separate control of each engine; and

Simultaneous control of all engines.

Each intermediate position of the mixture controls that corresponds to a normal operating setting must be identifiable by feel and sight.

The mixture controls must be accessible to both pilots. However, if there is a separate flight engineer station with a control panel, the controls need be accessible only to the flight engineer.