From a resilience POV, my guess would be that failure of any one blade would botch the system overall. Maybe that is why many diagrams show them installed in pairs. (I would guess each operates in a different direction for angular momentum reasons.) I have no idea about overall reliability.
For marine applications dual drives are common as it enables better rotational control for maneuvering. The redundancy aspect is also a factor, but moreso for applications where you are going to be far from shore. For tugboat and ferry type applications, where these drives are most common, that is less of a concern.
I think you are right, One only provides directional thrust, a pair would be needed for rotational thrust.
Most traditional tugs have a pair of screws for just this reason. Not so much to turn but by applying differential thrust they can pull sideways. A vector drive like this will vastly increase the envelope of possible pull conditions.
They are installed in pairs and work together with bow thrusters to allow for dynamic positioning, the RV Falkor (too) has them for this reason.