A fan has analysed the GTA 6 leak files and noticed that Jason and Lucia in the nightclub are not acting like NPCs. The characters in the footage move as if they were being controlled by two real players.
This feature makes many gamers think that GTA 6 will have co-op playthrough for Jason and Lucia. Also in the leaked footage we saw the scene of cafe robbery and police chase, where both main characters perform their tasks. We can assume that co-op would be ideal for such missions as well as for exploring the world together.