void setup() {
    pinMode(10, OUTPUT);
    pinMode(11, OUTPUT);
}

void loop() { 
    for (int i = 0; i <= 255; i++) {
        analogWrite(10, i);
        //delay(1); 
        for (int j = 255; j >= 0; j--) {
            analogWrite(11, j);
            //delay(1);
        }
    }
    for (int i = 0; i <= 255; i++) {
        analogWrite(11, i);
        //delay(1);
        for (int j = 255; j >= 0; j--) {
            analogWrite(10, j);
            //delay(1);
        }
    }
}