Electrical – Replace a 500Kohm potentiometer with a digital one

Probably not, practically.

It sounds like you have thyristor phase controllers and the pots would not only have to be inconveniently high voltage and power, but also you'd need to add isolation from the mains for safety.

You'd be better off (and much safer) finding a motor speed control that has an isolated low-level control input.

Edit: Speed controls of this type energize the field coil (if any) with a constant voltage and control timing of pulses to the thyristors supplying the armature based on the input speed command signal and motor current (to maintain a more constant speed). By controlling the timing relative to mains zero crossing, the average armature current can be controlled, and by measuring the armature current (a proxy for torque) the speed can be regulated fairly well without a tachometer.

For a controller with an external low-level input it would be designed from the ground up to use low level signals and something like an optocoupler or a small transformer would be used to transfer the control signal (perhaps 0-10V) to the mains circuitry using a linear optocoupler or a PWM scheme.

Trying to add a digital pot to one of these things (in my experience) would be like trying to add a digital pot to a wall light dimmer- really totally unsuitable and you're better off starting from scratch, with a much more complex design.

If I connect only one POT and motor controller all works fine, if I connect two, they burn out :( ?

Your circuit may have a ground loop. The potentiometers are connected to 'ground' on each speed controller, but these two grounds are only joined together via the negative wires going to the battery. When a motor draws current a voltage drop will occur across its negative wire, and then the two 'grounds' will not be at the same voltage.

On 48V a 500W DC motor will draw peak currents greatly exceeding 10A, which could produce spikes of several volts in the controller negative wire. The MCP41010 will blow up if the voltage on any pot terminal goes below -0.6V or above +6V (relative to Vss). Each MCP41010's pot is connected to a separate ground at the controller end, but their Vss pins are both connected to Arduino ground. Therefore they are exposed to any difference in ground potentials.

To eliminate this ground difference you have two options.

1. Join the two controller negative terminals together with a very short thick wire. At the Arduino end, connect both pot ground wires to Arduino ground. This may not be sufficient if the motor currents are very large.

2. Pass the SPI signals through opto-couplers, and power each MCP41010 from its controller's +5V and GND. This is the safest option because it isolates the grounds from each other, so any voltage differences that may occur between them are irrelevant.