Dualled.c

int green_led = 11; // 11 and 10 are pwm pins
int red_led = 10;
int max_brightness = 255;
int brightness_step = 5;
int delay_time = 20;

void setup()  { 
} 

void loop()  { 
  // fade in from min to max
  for(int fadeValue = 0 ; fadeValue <= max_brightness; fadeValue += brightness_step) { 
    analogWrite(green_led, fadeValue);
    analogWrite(red_led, max_brightness - fadeValue);
    delay(delay_time);                            
  } 

  // fade out from max to min
  for(int fadeValue = max_brightness ; fadeValue >= 0; fadeValue -= brightness_step) { 
    analogWrite(green_led, fadeValue);         
    analogWrite(red_led, max_brightness - fadeValue);
    delay(delay_time);                            
  } 
}