Hmmm... you mean coaxial props, or a conventional twin screw setup?
Most ships use a single large slow turning prop (85-105 RPM) and it is all about fuel efficiency, which can make or break a voyage in a highly competitive industry. However the length to beam is quite high and so side force at sea speed is not really noticeable. Also s sea peed (cruise speed) is considerably less than theoretical hull speed. Compared to small displacement vessels, for its length a ship is just idling along. Still the argument can be made that a single thrust cone if you will, is more powerful than two thrust cones sharing the same energy input. Simple geometry.
But OTOH, twin screws eliminate the inefficiency caused by side force, which is felt more in a small beamy vessel. And if a lot of maneuvering is to be done relative to actual cruising time, then twin screws are definitely worth considering. If they rotate inward, i.e. the port screw is right hand and the starboard is left hand, it is easy to crabwalk laterally without a bow thruster, whether making headway, sternway, or no way but sideways. And if they turn outward, pivoting maneuvers are extremely easy and effective, though with outboard-turning wheels, a bow thruster becomes much more desireable.
Another consideration is redundancy. A twin screw vessel with one shaft inoperative can still make way, just not as efficiently as a completely operational twin screw vessel. Redundancy is particularly important in yachts, where planned maintenance and emergency parts replacement inventory can be less than professional grade.
All in all, I think if building an e-boat from scratch, and it is too big to row or paddle, go with twin screws. Otherwise, single. And for a repower, go with the same configuration as original, rather than install two shaft tubes, two rudders, etc.
Coaxial props have potential for seriously high efficiency, I think, but the drive train has added complexity. The ability to selectively use only one prop for maneuvering would allow greater maneuverability through the selective use of side force, vs a single screw, however. Otherwise, cruising with both props turning, the side forces would cancel out, as would torque on the hull. And you still end up with a single "thrust cone".
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