The animation for 2-handed weapon are used. But the weapon in your left hand is used in the animation you'll deal damage and normal 1.5x strength modifier.
So you can use that longsword in two hands.
Not sure how hard it would be, I suspect it wouldn't be that hard. You're not adding any animations, just have to have a "hotpoint" on the hilt of the weapon where the animation would put the hands when swinging.
It's a standard pen and paper rule. Any slashing or crushing weapon can be used two-handed.