A good way to test it take a unit of Companions (good charge) and mod a unit with no shield to have 63 defensive ability (and preferably a low mass). Then present the Companions with the left side in a very deep formation so that they all hit the side. If the Companions penetrate deep or even just kills a fairly largen number of men, then Jerome is right and we need to revaluate every single unit, if it only kills a few men and is killed itself then we know that defensive ability is applied to both sides (perhaps not equally but at least the left is not screwed over).