If I understand the question, you only have have front/back bodice and front/back skirt piece…is this correct? You’re only increasing on the side seam of the pattern, not on the side that is “cut on fold” Because we only have 2 total side seams, you’re really only increasing the pattern on 2 sides. I know it’s confusing because you’re only doing it once per pattern, but, remember, the “cut on fold” means you’re fabric piece is folded in half, therefore, it takes care of both side seams. In order to make the distribution more even we split the amount we need to increase between the front and back patterns. If I alter the pattern by adding 1/2″ to the side seam, I’m adding a total then of 1″ to the front pattern and 1″ to the back pattern for a total of a 2″ increase, or 1/2″ X 4 = 2″. If I do the increase the bodice, then I must do the same increase to the skirt portion, because when these two elements are sewn together we want to make sure they match. I hope this helps and not make things more confusing.