Well, like you've pointed out, duel-wielders work best as skirmishers, harrying the flanks of the enemy formations while the shields are focused in the center. Basically, duel-wielders at their most effective act as calvary with increased mobility and speed than shield or spear users. If the spears are kept near the shields, when the duel-wielders distract and engage the enemy, the spears and shields should have a shot at an unprotected side.