Cheapest/quickest way to power 2 x 12v fans from 24v baseboard fan socket?
If the two fans have the same Power ratings they will split the voltage equally.
ADDA AD0824HS-A71GL 24V 0.16A 3.84W
If not equal then the lower power one will run at higher V and RPM until it’s incremental impedance matches the other.
Miniature BLDC Fan motors are designed more like controlled voltage/current drivers wto look like a resistor.  They have very low starting torque as a result but torque rises with current and Power out matches the DC power square law consumption.  They use precision Hall sensors to trigger each phase of the commutation and use PWM to reduce the average current as a ratio of the applied DC voltage.

i.e. I is proportional to V.
Hence a BLDC fan motor behaves like a resistor.

The current noise is LC filtered to reduce it somewhat. So the input Voltage splitting or divider ratio depends on the fans being matched for power or voltage and current specs.


Note the linear slope of current with horizontal voltage and the f(x) quadratic equation has a 0 linear term for x.

I have done this successfully with 2 matched 24V fans on 48V supply with temp controlled variable LM317 current limiter using transistor to Vadj. This was for high volume production.
I found that the linear current speed control method was more stable than using PWM for speed control here due to aliasing.
Here are some other examples.