I don't think they would ever fight - the rhubarb is really quite passive and would only fight if it was "forced" whereas the celery is all "heart". They both tend to go a bit limp so the fight wouldn't be up to much anyway :)
neither of you suprisingly havent mentioned that a rhubarb has a big leaf at the top. Maybe the rhubarb could fart and waft it with the leaf towards the celery and while the celery was temporarily dazzled the rhubarb could plant the celery in the stomach snapping it cleanly in half. Do you think this is possible?