First of all if your ship is fully loaded with crew then you have plenty of crew to reload and fire on both broadsides. This would only make sense if you were lacking crew, so for example you are sailing with half of the max crew or with a skeleton crew, that can only concentrate on one thing at a time. Most of the ships in that age weren't sailing with full crew, but for example a battleship was, so that in that age was able to fire and reload both broadsides.