Hero Duo is Archero's Co-op Mode where multiple players can work together to conquer 100 levels. It can be accessed from the Events tab daily, and allows 10 attempts per 24-hour period to beat the challenge, but only the first 3 attempts give rewards. This mode awards Gems, Gold, and Experience.
One player must act as the host and create a new room, which generates a 5-digit Room Code that other players may input on their device to join the instance.