unsigned long tx = 0, ty = 0;
unsigned long time1, time2;
int stat_led1 = LOW, stat_led2 = LOW;

void setup() {
  pinMode(2, OUTPUT);
  pinMode(4, OUTPUT);
}

void loop() {
  digitalWrite(2, stat_led1);
  digitalWrite(4, stat_led2);
  time1 = millis();
  time2 = millis();

    if(time1 - tx == 500){
      tx = time1;
      stat_led1=!stat_led1;
    }

    if(time2 - ty == 500){
      ty = time2;
      stat_led2=!stat_led2;
    }

}