This doesn't make any difference with straight, double-edged swords. But it's a noticeable issue for curved, single-edged swords.

When combining the two swords to form the twinblade (thief sword, two-bladed sword, whatever), both edges are facing the same direction. This makes spinning attacks ineffective. It just looks weird.

The best example of this in the benchmark is the weapon you receive using the Tantalus set.



Compare this with similar weapons from the material that clearly inspires this weapon, like FF9's The Ogre or Masamune. Or any Swallow from Chrono Cross. It is clear to see that one of the blades needs to be turned around.





This likely won't catch as much attention as the glaring issues with lalafell smiles and similar in the benchmark graphics update. But it is important.