Let's get some numbers into this. Say I have two units of strength X against a single opponent unit of strength Y (ignore promotions for now). Let's assume all units are melee, for the sake of simplicity. Now I can either attack the enemy twice with my units, or merge my two units into an army with a strength of X+10 and then attack once. In which case will my attack inflict more damage, and does it depend on the difference X-Y? At a first glance, I would expect two attacks are better against weaker units (X-Y >0) and an army is better against stronger ones (X-Y < 0). I wonder if math works this way too.