What you'll do is get THM and CNJ to 15 and ACN to 30. From there you can accept both quests to get both jobs. However, as a recommendation or two, get THM to 26 for Swiftcast which will benefit both jobs as a cross-class skill.. Also try to pick which one you want to focus on as they will share the skill points. If you want to main SCH then make sure to allocate more points to mind than Int. If you're going SMN, then allocate more points to Int rather than mind. Here's hoping you enjoy the job lines!

As a sidenote, once both are unlocked, both go off of the same experience bar and will level up at the same time. This means you don't need to go back and level SCH up to your SMN level and vice versa. It's a neat thing but you still need to watch where you put skill points.