int led1 = 12;
int led2 = 9;

void setup() {
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
}

void loop() {
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(led1, brightness);
    analogWrite(led2, 255 - brightness);
    delay(10);
  }

  delay(1000);

  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(led1, brightness);
    analogWrite(led2, 255 - brightness);
    delay(10);
  }

  delay(1000);
}