An double weapon is probably the superior choice. All the weapon specific feats apply to both ends, and you can use it as a two handed, or as a double weapon, depending on the situation.
Generally, two katanas is not an optimal choice because it's not a light weapon, so even with all the feats, you have a -4 to all attacks. That being said, When converting a second edition character, we just accepted the penalties and did it anyways. Turns out by level 12 or so, he didn't miss the -4 too much and was still a deadly fighter. If something was too hard to hit, he just put one sword away and used the other two handed.
Troll