I second Palom and Porom, definitely!
Also, ToroSonyCat's idea is pretty cool about the same character in different outfits. Another example following that route would be, say, to do two versions of Sora from Kingdom Hearts, or normal Garnet and Trance Garnet from FF9. Sooo many possibilities with that idea!
This thread makes me want to do more actual twin cosplays with my twin. XD I think we've only cosplayed related characters like, once. XD
Next Con: Anime Matsuri!
Simon Petrikov - Adventure Time
Jack Frost - Rise of the Guardians
Sandman - Rise of the Guardians