My idea is however they get their powers, they are merged into a single body but when they go to fight crime, supervillians, alien invasions, etc they split apart but as soon as they are done they go back to a single body. Kind of a reverse Firestorm situation. Also it is a boy and a girl, when in civilian form the guy is in control of the girl's body but the girl is always there to give advance to nag, etc. Also the guy while stuck in the girl's body is mentally attracted to girls but the body is attracted to guys, so he is screwed romantically.