Do I have to prep for both exams separately?

Good news: not at all. Studying for the SAT will improve your ACT score, and vice versa. If you improve your material knowledge for the SAT, it’ll apply directly to the ACT with few exceptions, and vice versa. The strategies for both tests are also nearly identical – the skillsets you learn when preparing for the ACT Reading section will apply directly to the SAT Reading section. While you’ll need to prepare for the format differences between the exams, and study the precise directions and style of both tests, about 85% of anything you study for one test will apply directly to the other.