The game is played the same as a 3 player game, but 1 player will be controlling 2 Treasure Hunters separately, having to collect 2 sets of Treasures each (2 Treasures of each color, 10 Treasures in total). All turns are made in the exact same order and each Treasure Hunter takes their turn separately.
The only other change to the rules is that the Treasure Hunter Player may collect any Treasure with any Treasure Hunter.