What would happen to the rudder on an A320 if both FAC systems failed?
The A320 has a mechanical link from the rudder pedals to the hydraulic rudder actuators. Computer-generated rudder commands for yaw damping and turn coordination are added to the mechanical signal from the rudder pedals, but in the event of a two-sided computer failure the rudder can still be controlled.

The elevator trim wheels are also mechanically connected, so rudimentary control of the aircraft remains even in the event of a total electronic failure, as long as there's hydraulic power.