I have a double alley and I will never recommend it to anyone.  Cattle like to follow the leader and once they start down one side they all have to follow down the same side.  They don't tend to like to walk in two abreast.  Add to that annoyance the odd phenomenon that the gate at the front of the double alley will almost always be swung the wrong way.  I wouldn't use anything more than half sheets and for the numbers that you are talking I would not go much more than a single 30' alley.