for reference, here is my code :

#include "DFRobot_GP8403.h"

DFRobot_GP8403 dac(&Wire,0x58);

DFRobot_GP8403 dac2(&Wire,0x5F);

int StateMachine = 0;

// Interrupt Service Routine (ISR)

void changeState ()

 {

 StateMachine++;

 if(StateMachine>4)

   {

     StateMachine=9;

   }

    Serial.print("Machine State = ");

    Serial.println(StateMachine);

 }

void resetState ()

 {    

   Serial.println("reset");

 StateMachine=0;

 }

void setup() {

 Serial.begin(115200);

 while(dac.begin()!=0){

   Serial.println("init error 1");

   delay(500);

  }

   while(dac2.begin()!=0){

   Serial.println("init error 2");

   delay(500);

  }

 Serial.println("init succeed");

 dac.setDACOutRange(dac.eOutputRange10V);//Set the output range as 0-10V

 dac2.setDACOutRange(dac2.eOutputRange10V);//Set the output range as 0-10V

 attachInterrupt(digitalPinToInterrupt(5), changeState, RISING);

 attachInterrupt(digitalPinToInterrupt(4), resetState, RISING);  

}

void loop(){

switch (StateMachine) {

 case 1:

   dac.setDACOutVoltage(2000,0); // OUT0 channel 1

   dac.setDACOutVoltage(2000,1); // OUT1 channel 1

   dac2.setDACOutVoltage(2000,0); // OUT0 channel 2

   break;

 case 2:

   dac.setDACOutVoltage(4000,0); // OUT0 channel 1

   dac.setDACOutVoltage(4000,1); // OUT1 channel 1

   dac2.setDACOutVoltage(4000,0); // OUT0 channel 2

   break;

 default:

   dac.setDACOutVoltage(0,0); // OUT0 channel 1

   dac.setDACOutVoltage(0,1); // OUT1 channel 1

   dac2.setDACOutVoltage(0,0); // OUT0 channel 2

   break;

}

 dac.store();

 dac2.store();

}