There would need to be a way to ensure the coupons are removed from each player at the same time, so that, if one player decides to pull out, the other loses their coupons. Or, if one player pulls out of the match, the other's coupons are returned to him. I frankly prefer the first option, since I foresee many annoying bails.
Another feature we could add is more than one private duelling arena to reduce the waiting time on matches.