int led1Pin = 9;    // LED 1 connected to PWM pin 9
int led2Pin = 10;   // LED 2 connected to PWM pin 10
int brightness = 0; // Initial brightness
int fadeAmount = 5; // Amount to fade the LEDs by

void setup() {
  pinMode(led1Pin, OUTPUT); // Set pin 9 as an output
  pinMode(led2Pin, OUTPUT); // Set pin 10 as an output
}

void loop() {
  // Set the brightness of LED 1 to be increasing:
  analogWrite(led1Pin, brightness);
  // Set the brightness of LED 2 to be decreasing:
  analogWrite(led2Pin, 255 - brightness);
  
  // Change the brightness for next time through the loop:
  brightness = brightness + fadeAmount;
  
  // Reverse the direction of the fading at the ends of the fade:
  if (brightness <= 0 || brightness >= 255) {
    fadeAmount = -fadeAmount;
  }
  
  // Wait for 10 milliseconds to see the dimming effect
  delay(10);
}