Fan speed control with resistor
The second fan is not connected in parallel with the first. It is connected directly to the neutral while the path to the neutral from the first fan goes through the thermal switch. You should make sure that the second fan is suitable for connection to the mains without an external thermal switch.

When the speed of a fan is reduced, the power required to drive it is greatly reduced. Power is proportional to speed cubed. That means reducing the speed to 2/3 of full speed reduces the power to 30% of full power. However, that calculation does not fully take losses into account.

In addition, the voltage drop and current calculations are not a simple matter because of inductance in the fan.

The bottom line is that it will not really be possible for you to verify if it is ok to add the second fan without connecting it and observing if the resistor gets too hot.