Is it a good idea to prepare for the SAT and ACT simultaneously? What if, for instance, I want to take the ACT in September and the SAT in October? Or even if I want to take both the SAT and ACT in June? Is this doable, or is this a terrible idea?
If you're taking either test for the first time, I'd advise you to stick for studying for one or the other, rather than both. Part of preparing for both the SAT and ACT is not just knowing the material, but understanding the test structure and practicing taking the test under realistic conditions. Neither test is particularly short (ACT with Writing is around 215 minutes, while the SAT with essay clocks in at 230 minutes), so it can be difficult to take multiple realistic practice tests when preparing for just the SAT or ACT, let alone both. Plus, keep in mind that there's no rule that says you must take both the SAT and ACT - no school's going to require scores from both tests for college admission.
If you have to take both tests in a relatively short period of time (due to fast-approaching deadlines), I would recommend taking the tests a month apart (e.g. a September ACT and an October SAT), rather than in the same month (e.g. the June 4th SAT and June 11th ACT).