You can but you shouldn’t.
Both have different content so you should only study for 1.
I’d take a practice for each (with enough sleep), and go all in on the one you do better at
you can totally submit both, I'm more just saying if you bomb out one you can just not submit it to universities and they won't know about it. when I took the ACT it was I think $50? I didn't study for it but I studied for SAT, I got a 1320 SAT and 32 ACT and my ACT percentile was higher so I submitted only ACT to universities. I think Laundry's advice on only studying for one is good advice and you shouldn't stress yourself out about taking both, just take the other anyways and if you do better it only benefits you. For me the "other one" was the ACT as taking the SAT was mandatory at my High School.
99.9% ur gonna do better on the one you studied for, so it doesn’t make that much sense to take the SAT and the ACT once instead of doing your preferred one twice
The big problem with this though is college admissions. Some (not that many tho) colleges require you to submit EVERY score you get, so your gonna have an egregiously bad score on there because you didn’t study for it.
It was like it when I graduated and also from what I'm seeing online it's still that you do get to choose for almost every college what score you submit. Unless 229fn has a specific college they really want to get into that requires all scores, which I'd double check before taking any test, I'd take both. Most people I know it was very 50/50 between SAT/ACT studying or not, because science was a larger factor in ACT some STEM people got a higher score on it without studying. It wouldn't be a loss to take both, nor take both multiple times (within financial reason for your family). I know some people who took the SAT 3-4 times.
Edit to add: It was heavily pushed in my area to take both tests and take them as many times as you can. The questions do vary so if you get a good set of questions you know, have a random good day, got good sleep, you get the idea, but you really can get a better score than you even expect. Do it within reason mentally and financially but there is no reason not to otherwise imho. If anything, since me and Laundry have such conflicting views, ask a professional at your school for their opinion and advice on the topic.