int ledPin1 = D2;
int ledPin2 = D3;
// create a variable to store the
// current brightness of the LED
int ledValue = 0;

void setup() {
//Register our Particle function to allow
   // Control of the LED
   Particle.function("led", ledControl);

   Particle.variable("brightness", ledValue);

   // Set up pin for output
   pinMode(ledPin2, OUTPUT);

   pinMode(ledPin1, OUTPUT);
}

void loop() {

}

int ledControl(String command)
{
    // Convert the passed variable to an integer
   ledValue = command.toInt();

   // Check it is a valid number
   if( ledValue > 255 ) return -1;
   if( ledValue < 0 ) return -1;
   if (ledValue < 255){
        int led2Brightnes= (255 - ledValue);
        for(int j=led2Brightnes; j>=0; j--){
        analogWrite(ledPin2, j);       
        delay(10);
       }
       for(int i=ledValue; i>=0; i--){
        analogWrite(ledPin1, i);       
        delay(10);
       }
   }  

   // Return 1 to say completed successfully
   return 1;
}