From my understanding using a spear with two hands is more effective then one handed spears. The Macedonians used small shoulder mounted shields that allowed them to effectively duel wield the sarissa with a shield. This is the most I've ever read about this tactic being used and I can't find any information as to why. At a later point in history someone invented the bouche which would allow spears and lances to be balanced and held in a groove on the side of the shield, but they were used mostly for jousting. Why did no one ever use these specially designed shields to duel wield a two handed spear and shield in actual combat other than the Macedonians? It seems like it could have been a huge advantage for several armies throughout history like it was for them.