Can I have two regulator for wall fan - one default one and second I externally connect at switch board
With the most common products, the added regulator work by reducing the supply voltage while the built-in regulator works by changing the capacitor value or connection point to the motor winding. The two can work together, but attempting to make the slowest speed very much slower may not work very well. The fan speed may vary and it may be difficult to adjust the fan to a slow speed without having is stop completely. It will probably work best if the fan regulator is set to the fastest speed and the added regulator is used only to reduce the speed from that point.