To understand that, it helps to know that they will both complement eachother, and improve the overall precision of the calibre.
The Duplex swan-neck fine adjustment is connected to the balance by means of the stud-holder and enables regulation of the anchor symmetry. Adjusting the adjustment screw allows one to turn the entire balance so that the impulse pin is centred in the anchor fork, which optimises the insertion of the pallets in the escape wheel.
Thus on the right side one can regulate the rate (V/N) and on the left the anchor symmetry – for extreme precision.