This idea as been discussed before but I think it was decided not to do this because it would require (well if you don't want it to look funky) the weapon model being turned around and this isn't possible in game...I think that was the conclusion.
On the note about the 2handers polearm mode
I think most pole arms should get a 2handed mode just to make it even and,
and I think most one handers should get a 2handed mode! I mean what weapon can you hold in one hand but not in two?
Of course leave some that don't have duel modes and maybe make the alternate mode a little worse than the originals.