There is a typo on the nemesis mat for Paradox of Myth and Bone. Its additional rules read, "When there are no nemesis cards in play or in either nemesis deck, the players win." This should read, "When there are no nemesis cards in play nor in either nemesis deck, the players win."
The win condition is that both nemesis decks are empty and there are no nemesis cards in play at the end of any turn, similar to the standard single nemesis rules.