unsigned long prevMillis = 0;
unsigned long prevMillis2 = 0;

// Counter (seconds)
int counter;
int counter2;

// Finite state machine
#define COUNTING 0
#define DISPLAY 1
int State;

#define COUNTING2 0
#define DISPLAY2 1
int State2;

const int ledPin =  LED_BUILTIN;
int ledState = LOW;
unsigned long previousMillis = 0;  

void setup() {
  Serial.begin(9600);
  
  pinMode(ledPin, OUTPUT);
  
  // Starting values
  counter = 10; // milisecond convert
  State = COUNTING;

  counter2 = 20; // milisecond convert
  State2 = COUNTING2;

  prevMillis = millis();
  prevMillis2 = millis();
}

void loop () {
  test();
  
  if (millis() - previousMillis >= 1000) {
    // save the last time you blinked the LED
    previousMillis = millis();

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(ledPin, ledState);
  }
}

void test() {
  switch(State) {
    case COUNTING:
      // the device you want to turn on first
      if(millis() - prevMillis >= 1000) {
        prevMillis = millis();
          Serial.println(--counter);
  
        if(counter <= 0) {
          State = DISPLAY;
        }
      }
      break;
    case DISPLAY:
      switch(State2) {
        case COUNTING2:
          // the next device after the first is done
          if(millis() - prevMillis2 >= 1000) {
            prevMillis2 = millis();
              Serial.println(--counter2);

            if(counter2 <= 0) {
              State2 = DISPLAY2;
            }
          }
          break;
        case DISPLAY2:
          counter = 10;
          prevMillis = millis();
          State = COUNTING;
          break;
      }
      break;
  }
}