I think this subject goes a little "deeper". If my old memory serves me correct I believe the theory is that a boat's transom actually moves left or right because of the propeller's depth in the water. Water near the surface has less pressure, and that pressure increases the deeper the propeller gets.
The bottom line is that if a propeller turns counterclockwise there is less sideways pressure near the surface and the opposite side of the propeller (the bottom) has more sideways pressure forcing the stern of the boat's transom to go left (port). For clockwise rotation the opposite is true.
So using a counterclockwise prop will cause the stern to move to the left and this is handy when docking a boat to the port or left side of the boat. By steering slightly to the left while going forward slowly the whole boat will actually slide sideways slightly left. This will make it easier to dock to the port side of the boat. If you want to make it easier to dock to the right side you would use a clockwise turning propeller. (Most use the left, port, side).
As for dual props turning in opposite directions the sideways forces are equalized by each propeller. However if the left prop turns counterclockwise and the right prop turns clockwise there is more afterward thrust, again due to the deeper pressure.
One thing to watch for when using brushed motors is most of them are more efficient in one direction than the other. This is due to the brushes being offset inside the motor. In my experience it seems the RPM is also greater due to this fact. So using two identical motors one on each prop, may cause one prop to turn slightly faster than the other.