// now do the same for the other sensor and LED:
  leftSensorValue = analogRead(A1); // read the pot value

  // map the sensor value to the brightness again. No need to
  // declare the variable again, since you did so above:
  brightness = map(leftSensorValue, 400, 900, 0, 255); 

  analogWrite(greenLED, brightness);  // set the LED brightness with the result
  Serial.println(leftSensorValue);   // print the sensor value back to the serial monitor
}