Enable PVP for some players

In a multiplayer survival server, we'd like to have PVP off unless two players decide to fight.
Is there some (Forge) mod or plugin that can set players to PVP on at will, or is the best way to have an admin spawn a few command blocks, set all players on a no friendly-fire team, and make the command block set the player who hit it to a different team (in adventure mode, so he can't escape the arena)?
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