Split and Reunion

A character or an entire group gets split. It can be a small rebellion of characters thinking differently or a Split Personality of a character creating his Evil Twin. The two entities created are so completely opposite that it's hard to believe they have something in common. Then, however, they reunite. The reunion results in creating something more perfect than the original entity. The main plot is often put in the time when the entities split. The reunion would be connected with the climax or resolution of the plot. The moment of the split would be often revealed in retrospection. Revealing this fact would be a plot twist.

Anime and Manga

 * In Dragon Ball Piccolo is a result of Kami-sama's struggle against inner evil. They live separately as foes for a time, but then Piccolo absorbes Kami-sama becoming more powerful.

Comic Books

 * The entire Marvel Universe has undergone this since Avengers Disassembled. Civil War split The Avengers into two seperate teams led by Captain America (comics) & Iron Man respectively, with most of the other superheroes picking a side. It took an alien invasion, Norman Osborn becoming the world's supercop, Captain America's resurrection and Iron Man having his brain rebooted from a back-up that he made prior to Civil War to reunite the various heroes against a common foe.
 * On top of that, the X-Men have generally stayed apart from the rest of the characters since House of M ended with most mutants being depowered & as of this entry, are now at the point that the X-Men are being split in two & fighting each other.

Film

 * In Dark Crystal the Skeksis and urRa are depicted as completely opposite. From the first moments of film however we can see some synmetries between them.

Video Games

 * In the second part of Star Control Pkunk are revealed as the offspring of Yehat, with Pkunk being New Age Retro Hippies and Yehat Proud Warrior Race Guys. At the end of the game however