Feeding twice a day is a bit much in my mind. But then again I feed rather sparingly. Sometimes skiping days.
One way to determin if your feeding to much is water quality. If you dont have or dont start having issues with excessive nitrates or show signs of ammonia or nitrite and you dont end up with algae issues then your not overfeeding your tank.
Its as much wanting ot keep from overfeeding the tank as it is to keep from overfeeding the fish.