There are no hard-and-fast rules for multi-player duels. My friend and I have only discussed these rules: we do not have a third Wizard/Witch to duel with. We wanted it to resemble a real-life duel, which can have tag-teams, betrayal, and free for all chaos.
-You may choose which opponent is dealt damage by which creature (you only have 1 creature, so it can only damage 1 Wizard/Witch, though you cannot split up an individual creature's damage).
-When it says "opponent" assume for multiplayer that it means "an opponent you choose." After all, you cast spells (typically) at one opponent at a time.
-Adventures can be tricky but assume all opponents are affected (similar to a location) so any opponent may solve adventures.
-As the Wizard/Witch, you always get to choose where damage goes, which opponent a card effects.