Great question! This really depends on you. I recommend taking a full length practice ACT
and SAT in a proctored/test-like environment - preferrably early on a Saturday morning! Then, based on your results and how you felt while taking each practice test, you decide which test to prepare for and take first! If aftering taking your first test you decide to try the other test, then you can. Preparing for either test first will teach you strategies that apply to the other test, but I do recommend doing structured preparation (online or in-person) for the other test if you decide to take it!