...both protagonists fighting each other's antagonists.
Let's take LeonhartFour's mock bracket for an example:
The first match was (1)Link/Ganondorf vs. (16)Sly Cooper/Dr. M
So, the staff would write Link fighting Dr. M while Sly Cooper fights Ganondorf. In the end, Link would defeat Dr. M and Ganondorf would defeat Sly. And so on, so forth.
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Nominate Cosmos and Chaos for the Rivalry Contest.
@GenesisTwilight: Kinda. It's not as epic as before, but it's been still pretty good. Lately, Cloud and Squall is in charge of writing fanfics of this kind, archiving it on the Bounce It board. Lately, the characters are scaling Tartarus, facing all kinds of challenges instead of fighting each other.
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Nominate Cosmos and Chaos for the Rivalry Contest.