You can control multiple devices with the same PWM signal, but treating it as a single device - you will get mixed RPM readings, though (summarized from all devices which will jump for example from 300RPM to 5000)

If you want to control each device independently via PWM then you will need another extra controllers.

1. Cables
PWM same as RPM signals are one direction only.

PWM wire provides only sending signal from the generator to the recievers (for example fans)
Each RPM signa is generated individually, and it is just a geenrated number of voltage peaks that are counted by the reciever. When you join the multiple cables together then you will get mixed results, resulting in bizarre readings like in one second you get 400rpm, whilein the second moment you get 5000 and it constantly fluctuates, although all devices run on the same voltage, generating the same RPM (well with small variations).

That's why you would have to mix the cables

PWM split to each cable to the fan/device.
RPM signal signa from each device individually send back to the specific RPM reading channel.

The easiest is to remove RPM cable from the plug and put it to separate plug.
In the result you got fan/device with two plugs:
- 3 cables - voltage + ground + PWM control
- 1 cable - RPM signal

Aldhough it is advised to split the ground cable from the fan/device so that you got 2 separate plugs:
- 3 cables - voltage + ground + PWM control
- 2 cables - RPM signal + ground


Then you use Y splitters or other multisocket cable to split the first 4 cable cable.
The rest of the RPM cables goes to the recievers.

So you end up in a 2x more cables than devices

2. Splitting
I believe you can split it a large amount of times, because it is a signal control, which usually does not cause any electric current, because electronics reacts on the voltage.
I guess, you could easily hang 10 devices on it or more.

Bigger problem is the electric current and the wattage that can be drawed from the +12V line that is in that PWM jack.
Well, unless you also provide +12V from other sources, like in pumps: separate 12V, separate thin cables just form PWM signal.

Assuming that single fan eats 0.6W at 12V then that means it eats around 0.05A
While you can use about 1.25A on the aquaero 5 pro, that means you could run 1.25/0.05 = 25 devices on the PWM (for example on FAN1 or FAN4) without any external power supplies.