Although i never seen it that can ofcourse be true .
All pc powersupply's i seen so far were with 1 transformer With pwm usualy based on the tl494 or clones .
The feedback loop goes with a resistor from the 5 volt line to the tl431 .
Usualy the current through the feedback loop is about 1 mA which would make a 2k5 resistor to the Vref of the tl431 .
(Ofcourse thats a matter of choice of the designer so always check the value of the resistor so you can calculate the true amount of mA)
For 12 Volt that would mean you need to add a resistor of 12 Volt - 2.5 Volt / 1 mA = 9.5 Kohm .
By doing it this way you include the 12 volt into your feedback loop and it will now regulate both lines .
Just look up that resistor between the 5 Volt line and the Vref connection of the tl431 further its relative simple math