Two Stern Thrusters next to each other

Has there been and testing of two stern thrusters close together and what if any effects this causes.
I know some bow thrusters have two counter rotating props, does this make it more efficient?
Any information would be helpfull.

Thanks,
Lewardsail

 

Counter-rotating concentric props can (if properly designed) improve efficiency by reducing swirl in the water stream exiting the props. With a single propeller the water stream exiting the prop is swirling and the swirl does not contribute to thrust.

The propellers must be counter-roatating, concentric and properly designed to reduce the swirl and improve efficiency. Otherwise efficiency may be worse than with a single propeller.

 

Do you mean 2 tunnels or 2 azimuthing thrusters?
I think the other advantage of duo props is small diameter to equal the thrust of a larger diameter