I think a multiplayer duel mode wouldn't work as well for Divinity, mostly because of the limited party size. The items, skills, spells and such are all based on a long-running game, not a five minute melee. A warrior is really the only one with a chance, unless a wizard kills them before they get too close.
Has anyone played Daiblo I multiplayer? How well did that work?
Perhaps a us-vs.-them mission system would be better, like capture the flag?
(Don't worry about our responses. You just got three anti-multiplayers first. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />)