I mean spears are currently a little bit weak and even i'm telling this when i'm not really fan of spears. You can consider to allow thrust attacks with spears while blocking a direction with shield. This will bring new balance problems of course. Spearmen will be more defensive aganist ranged weapons. I mostly fear for one handed weapon powerlessness in this situation. And one more thing; shield walls can turn into phalanxes with this which is even more terrible.