Maru and Moro from xxxHOLIC -> Female
Sakura and Sakura from Tsubasa -> Female (One is a clone, the other is real, so fun parallel there)
Chii and Freya from Chobits -> Female
Hikaru and Kaoru from Ouran High School Host Club -> Male
Yuui and Fai from Tsubasa -> Male
Syaoran and 'Syaoran' - > Male (Same with Sakura/Sakura, one is a clone, the other real but this duo has some fun costumes.)
For some non-twin but corresponding costumes, you could go with;
Sumomo and Kotoko from Chobits -> Female and Persacoms. Whee!
The many identity crises of Sleeping Beauty/Aurora/Briar Rose
Kairi and Namine from Kingdom Hearts 2 -> Female
Tomoyo and Sakura from Card Captor Sakura -> Female
Kikyou and Kagome of Inuyasha -> Female (They share a soul and whatnot)
That's all I can think of at any rate. I'll probably remember and add more later.
Everything moved back to my original account (DarkWolfNite
) that I FINALLY remembered the password to after 5 or so years.... SO yeah, go there peeps! <33