The ACT science section is not about science. It is about data interpretation. It’s relatively easy for a bright, motivated kid who can concentrate to do well on it - they just need to do a bunch of ACT science sections for practice, so that they are familiar with all the ways that data can be presented on the test.
I suspect that it is easier to get a 36 on the ACT than to get a 1600 on the SAT. I also have heard that the English/Reading section of the ACT is easier than the English SAT section, so the ACT might be better for a non-native speaker of English.
Only reason to do both is if student is trying to figure out which one they can perform better on. Otherwise, a good score on either is enough.