That's what PWM is really all about . . .
Supply a full 12V to the device, either fan or pump, and then the PWM signal instructs the device to chop the full 12V supply into short bursts so that you can get a reduced speed without reducing the torque so much as with variable voltage control.
Having the splitter with a single 12V connection to the PSU, which it then distributes to the normal power pins of each fan headers, simplifies the wiring greatly, and means much less power has to go thru the main circuit board of the Aquaero.