we verified that Greatswords (and Large Longswords) cannot be offhanded with any combination of feats.
For a lrg greatsword in the offhand you're out of luck, however a lrg longsword can be wielded in the offhand with a race/feat combo. You just need a race with powerful build (Goliath for example) that can use weapons one size catagory larger at no penalty, then select the Oversized Two-Weapon Fighting feat (complete warrior book maybe?) and presto! you can now TWF with twin large longswords!