Basically, iron body and iron wall do not stack and it is matter of the order you use them. Say, you mastered iron body(weapon D +40) and iron wall(weapon D and magic D +20 each). If you use iron body first(now WD +40) and then use iron wall, now it is only WD+20 and MD+20.If you use iron wall first(now WD +20 and MD +20) and then use iron body, now it is WD +40 and MD +20. Most of the skills work like this.