Axial fans just don't generate the static pressure required to push air through ducts. They're fine for wafting large volumes around but useless for generating fast moving focused flows.
The design of the duct generates equally distributed flow to both nozzles. The runners are equal length and volume, while the larger volume from the outlet of the fan to the start of the runners acts as a low velocity, high pressure plenum.
You can use a LM2596 step down to regulate the supply voltage to a 12v fan on a 24v printer and still achieve full PWM speed control as it's the ground that's modulated.
If you get a half decent ball bearing 5015, then the duct is efficient enough that you will rarely need more than 50% fan speed and the noise is significantly reduced.