With two sets of One Deck Dungeon (or with the base set and an expansion), you can play as a four hero party. With four heroes, two encounter decks are shuffled together, and most things in the game are doubled.
Closed doors have two face-down cards, and when the party chooses to Enter a Room, they split up into two groups of two to face the two encounters. Each is resolved separately, as a 2P encounter.
The party chooses which order to resolve the encounters. A number of other changes are necessary, as well. â€¦