What they mean was most likely, if you level one of them, you level the other, since they're both are based on ACN. Just that. So for example, if you're level 48 SCH, when you equip your SMN stone, you'll be level 48 SMN. Just do both starting job quest when you got them. If you're like me, I just focus one the one I like more (SCH, though I also like SMN, I just like SCH more), but still gather some SMN equipments, and when I feel like playing the other one, I just use the stat reset item (really cheap from your grand company) to reassign my stats to a more proper one (from mnd to int, for example), and go blow stuff up as SMN.