The trouble with this is SE would have to go back and create whole new sword animations for GLA/PLD to do this, as well as having to recode the shield to remain on the player's back while the sword is drawn (as at the moment it's coded to be used mutually with the shield), a lot of time and effort that quite frankly could be better spent designing new classes.

Besides, this would conflict with the very description of GLA, where the class is specifically mentioned as being one-handed, and that some GLAs' 'like to keep their off hand free to focus on their sword arm', thus making two-handing swords contradictory.

Instead I think SE would be more likely to create a whole new 2-handed weapon type, either for GLA/PLD itself (as in, great swords), or else have it as a whole new class, which seems to be the popular choice.