Wow, that surprises me. I don't know how tripping with a two-bladed sword would be any different from tripping with a quarterstaff, but I also don't know much about two-bladed swords. So I guess it'd be, "All double weapons except for the two-bladed sword."
So if you take it for a weapon that already allows tripping, then it'd just be redundant. Unless you wanted to add, "If the weapon already allows you to make trip attacks, you now get a +2 bonus to the attempt." Which is similar to the Exotic Weapon Master ability that I mentioned earlier, if anyone bothered to look it up.