int redLED = 2;  
int yellowLED = 3;

void setup() {
  pinMode(redLED, OUTPUT);
  pinMode(yellowLED, OUTPUT);
}

void loop() {
  for (int k = 0; k <= 255; k++) {
  analogWrite(redLED, k);
  delay(15);
  analogWrite(yellowLED, 255 - k); 
  delay(15);
 }

  for (int k = 0; k <= 255; k++) 
  {
  analogWrite(redLED, 255 - k); 
  delay(15);
  analogWrite(yellowLED, k);
  delay(15);
  }
}