You can use polearms and spears as double weapons.
Benefit(s): Choose one weapon from the polearm or spear fighter weapon groups. While using this style, you grant the chosen weapon the double special weapon feature, using the weapon’s normal statistics for its main-hand end and the statistics of a light mace for its off-hand end.
A weapon wielded in this way loses the brace and reach special weapon features.
Pathfinder Player Companion: Weapon Master’s Handbook © 2015, Paizo Inc.; Authors: Alexander Augunas and David N. Ross.