a level 2 barbarian with the titan mauler archetype can use a two-handed weapon in one hand at a -2 penalty and is treated as a one-handed weapon for effects power attack, strength bonus to damage, and the like. a level 11 fighter with the two-weapon warrior archetype can use a one handed weapon as a light weapon. so would a level 13 character(11 fighter/2 barbarian) count a two-handed weapon such as a great sword as a light weapon?
RAW? Probably not. At my table, that wouldn't be remarkably overpowered of a combination at 13th level.
i mean the character would still be at a -4 to attack(-2 from the oversized weapon and -2 from two-weapon fighting)
Why would you do this when you could use a bastard sword for one feet and be able to get more damage output? Average damage for a Greatsword would be 7 average damge for a bastard sword is 5.5 and you would be at +2 to hit, plus with improved progression for Twin Blades and earlier access to your other abilities you would be much more likely to hit. I think if you are looking to optimize you characeter for damage you are better off dropping the barbarian and going straight fighter.
Edit: Not straight fighter but two weapon warrior.
Titan Mauler is one of those trap choices for people who like big numbers, just like Monkey Grip was in 3.5.
The weapon's damage dice are only a tiny portion of the overall damage you'll be dealing with a weapon that it's barely worth caring about.