Actually the Elevators each have four computer control possibilities and two hydraulic drive possibilities:
Control priority for each elevator and associated hydraulic system for each respective actuator is as follows:
Left Elevator:
PRIM 1: green
PRIM 2: blue
SEC 1: green
SEC 2: blue
Right Elevator
PRIM 1: green
PRIM 2: yellow
SEC 1: green
SEC 2: yellow
Stab trim is driven by blue and yellow hydraulic driven actuators (usually simultaneously), If one is out, then the other can drive it independently.
Hydraulic power to the two motors can be controlled by any one of three electric motors or mechanically through cables from the pitch trim wheels. The three electric motors are each controlled by a different PRIM.