Above illustration is the basic setup for the question. The scenario will be the following:
A potentiometer(providing 0 to 5V) is an analog input to A0. This potentiometer voltage is converted to PWM(pin9) which controls a fan M1. Which means the human controls directly the M1 fan speed.
This directly speed-controlled M1 fan creates air pressure on a sensor Sen1.
On the other hand I want M2 fan to reach to a speed(via pin10 PWM) such that Sen2 should output the same voltage as Sen1. Which means eventually voltage at A1 analog input should be equal to A2. So one can achieve same air pressures at Sen1 and Sen2.
I'm not experienced in PID or other type of controls. I would be glad if someone can help me to relate parameters and code this scenario.