I'm going to assume you're not willing to crossplay/dress so...
Lilly and Hanako or Shizune and Misha from Katawa Shoujo.
Patty and Liz from Soul Eater (Bonus if you can get a 3rd to be Kid)
Mariya and Matsurika from Maria Holic
Any two female Vocaloids
and the list goes on and on
Sitha Claus (Holiday Matsuri 15)
Pokemon, Gold/Ethan (Shado 12, Metro 13)
Batman, Black Mask (Khaotic 14, Metro 14 & 15, Holiday Matsuri 15)
Soul Eater, Spirit/Death Scythe (Shado 12, Metro 13 & 14)
DC Comics,The Question (Metro 15, Holiday Matsuri 15)
Durarara, Shizuo (Metro 15)
Steins;Gate, Okabe (Metro 15)
Noragami, Daikoku (Metro 15)
Dr.McNinja, Dr.McNinja (Holiday Matsuri 15)