Codenames duet keeps the basic elements of codenames, give one word clues to try to get someone to identify your agents among those on the table, but now you're working together as a team to find all of your agents. (Why you don't already know who your agents are is a question that CONGRESSIONAL investigators will get on your back about later!)
Collectively, you need to reveal all fifteen agents — without revealing either assassin or too many innocent bystanders — before time runs out in order to win the game. Either player can decide to give a one word clue to the other player, along with a number. Whoever receives the clue places a finger on a card to identify that Agent. If correct, they can attempt to identify another one. If they reveal as many as the number stated by the clue giver, then they can take one final guess, if desired. If they identify a bystander, then their guessing time ends. If they identify an assassin, you both lose.