But are they co (i..e. equal)-protagonists (as in Lethal Weapon, e.g. two equally essential main characters who have the same quest although each has different psychological needs)? Sam has the primary goal (to protect Molly), but Oda Mae as you point out has the arc. Oda Mae is a reluctant protagonist at best and has to be forced into action (as in Cutter's Way or The Fisher King or The Iliad) to fulfill someone else's quest and thereby achieve her own arc. It's an interesting twist on the usual structure--innovation within convention as you say.

A thought-provoking analysis.