Actually, Yuna uses Tidus' Caladbolg as her sword in the Dark Knight dress sphere, just as she uses the Brotherhood as a Warrior (she also uses Tidus's attack animation and victory pose, IIRC). Rikku uses Auron's Masamune as a Dark Knight.
A lot of the weapons in X-2 came from X. As a Black Mage, Yuna uses her Nirvana. Black Mage and White Mage weapons for all the characters are versions of Yuna's weapons from X. As Samarui, Rikku and Yuna both use weapons that Auron used in X. As a Warrior, Rikku uses another of Tidus' weapons. Paine gets the most unique weapons, which makes sense, as she never knew the rest of the team.