My experience is that there are negatives to having a wide formation - from having your unit split during fighting. I find that heavy infantry in 3 ranks do better than those in 2 - maybe because there are more men to replace those killed in the centre - so survivors in this part of the unit are less likely to be doubled? - i'm not sure what the reason is, but i don't generally use infantry in 2 lines anymore. Also, as said before, manouvering 2 rank units and getting a clean charge is quite hard in the heat of battle.