Yeah ... this fighting single-file is not all it's made out to be.  I kind of forgot that, by rules, neither of us can actually move at all (except to go all the way to the back) because one can't end their turn in a square occupied by another creature.  Unless you want to be generous and allow them to take turns simultaneously with 5-foot steps (switching places and avoiding attacks of opportunity), I'd say: Idise goes first and just gets out of there with a withdraw action, moving to the nearest unoccupied square, after which Zanthus moves up, and I'll edit my post.