void setup() {
  pinMode(5, OUTPUT);
  pinMode(9, OUTPUT);
  Serial.begin(9600);
}

void loop() {

  for (int a = 0; a <= 255; a++) {
    int n = 255 - a;

    analogWrite(5, a);
    analogWrite(9, n);

    Serial.print("a: ");
    Serial.print(a);
    Serial.print(" n: ");
    Serial.println(n);
    delay(1000);
  }
  for (int a = 255; a >= 0; a--) {
    int n = 255 - a;

    analogWrite(5, a);
    analogWrite(9, n);

    Serial.print("a: ");
    Serial.println(a);
    Serial.print(" n: ");
    Serial.println(n);

    delay(1000);
  }
}