The double property of this weapon has me uncertain. Can a character with the exotic weapon proficiency (Taiaha) wield a large version of this weapon in two hands? Would they then have the choice to use the hammer or spear end when they attack?
They have an oversized double weapon. You can do everything you could do with a normal double weapon with the added 'benefit' of taking a -2 to the rolls. So you could use one end, the other end or both ends.
Dot.