Sky,both props rotate in opposite directions,through a common but complex gearbox and propeller system,driving two propeller shafts,one inside the other...I am sure that each engine can then be `synchronised` with all the others to reduce a harmonic ` beat` that you get(drone) throughout the aircraft if one or more engines are running at slightly different RPM. This can be refined further by `synchrophasing the propellers` from the `master engine`.This is really a job best left to a `flight-engineer`/witch-doctor,as it invokes much voodoo,incantations,time of day,time of month,ascension of moon,periodic tables,and copious amounts of coffee/tea/pasties,etc...
I`m sure ``Shackman ` can elaborate further....my experience was limited from the C-130,and getting engineers to `get rid of that f£$%^& beat`..