Ok, I did a custom battle test... 1 unit of muskets vs 1 unit of muskets. The AI spread its muskets out in two rows while I used a square formation. Only the muskets in the front row will fire. When they finish shooting they will run to the back of the formation to reload so the next rank can fire. I can confirm that this does not work for muskets but even though it was only one test, the square formation beat the 2 rank formation. Hmmm...