When two Adventurers play, they take it in turns. Each player has a fixed number of turns and then the other player takes over. You can either explore as rivals or as allies. As rivals, it's probably best if you don't watch the other person's progress. The person whose round has ended should go away to wait for his next round. As allies, you still take it in turns, but anywhere you go your ally comes with you.
In fights with monsters, your combined weapon count is pitted against the monster. While you're allied, it's best if you both watch the screen all the time.
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We played this as a class at primary school, don't think we ever completed it though.
Wow! It was ambitious to play Xanadu Adventure at primary school. I don't even remember primary school, and I still can't figure it all out!: https://www.youtube.com/watch?v=dBvf5GeS7Ec