Chess variant with two kings on each side. Neither king can move into check, and both must escape check. If it is impossible to save both of them at once, it is a checkmate and that player loses the game.
That means that one can win by either checkmating one king, checkmating the other king, or attacking the two kings at once such that both cannot be saved (e.g. fork).
Otherwise the rules of chess apply, with the following differences:
Black checkmated one white king, and won.
White won, because both black kings are in check (fork), and there is no way to save both.
Black won. One white king is in check, and cannot escape, because then the other king would be exposed to the check.