Since you're also asking for groups of two, and if one of you don't mind crossplaying: Alexiel and Rosiel from Angel Sanctuary. Or, for a very rarely cosplayed couple, Integra Hellsing and her "sister" from the Hellsing anime, Laura. There's Aya and Ran Fujimiya from Weiss Kreuz (I love Weiss, so I'm biased). Or Ranma and, well, Ranma, from Ranma 1/2, which would be rather funny.
~Forget world peace. Visualize using your turn signal.~
"Don't engage in a battle of wits with an unarmed man." (Crawford, courtesy of Aoe, in this case)