Some of the Ivies will allow students who took the ACT with Writing to skip the Subject Tests. It depends where you are applying. Of the Ivies that offer Early Decision, Penn, Columbia, and Brown do not require Subject Tests from students who took the ACT. (For breaking news on Penn Subject Test recommendations … not requirements … see the bottom of this post.) At Cornell, specific Subject Tests are required by some “schools" within the university, but not by others, regardless of whether or not you took the ACT. Dartmouth does expect two Subject Tests except from students for whom the test fees present a financial hardship.
Note, however, that many students who apply to these hyper-selective institutions will submit strong Subject Test scores, whether they have to or not. The scores will show strength in areas (e.g., foreign language, history, bio, physics, chem) that the SAT and ACT don't cover. So if you send in ONLY your ACT score, you may be putting yourself at a slight disadvantage. On the other hand, if you send in crummy Subject Test results, you may be putting yourself at a GREATER disadvantage.
Thus, if your ED college is one that does require Subject Tests for ACT-takers, my advice would be to take another Subject Test (or two) but only if you believe that you are likely to do well.
If your college does NOT require Subject Tests for ACT-takers, you should only try another Subject Test if you have confidence that your score will be high (and taking a practice test at home is a good way to get a ballpark sense of where you stand).
However, if you have scored well on AP exams, this can suggest the same diversity of academic strength that good Subject Test scores will show. So, if you have good AP results and aren't required to send in Subject Tests, I recommend no further testing. I don't see any reason at all for you to take the SAT since you have a high ACT score.
By the way, I got my information about Subject Test requirements from a list that is compiled annually by Cigus Vanni, guidance counselor at Cherry Hill High School West in New Jersey. Each year he gathers data about testing policies and other critical (and confusing!) information and circulates it among thousands of colleagues via the National Association for College Admission Counseling forum. So a big thanks to him for going the extra mile for all of us.
I hope this helps, and here's one final piece of advice … probably better than anything that came before it:
You should double-check requirements on individual college Web sites or directly through admission offices. Do not rely on what you learn from an Internet “Dean" who has never actually been the dean of anything. 😯
Breaking news from U. of Pennsylvania:
Beginning with the 2015-2016 admissions cycle, the University of Pennsylvania will require all freshman applicants to submit the results of either the SAT or the ACT college entrance exams. In addition, Penn will recommend that each student submit the results of two SAT Subject Tests.
Applicants considering fields in STEM (science, technology, engineering and mathematics), are recommended to take the Math 2 SAT Subject Test and a science SAT Subject Test. Candidates applying to the Wharton School are recommended to take the Math 2 SAT Subject test. Candidates applying to Penn Nursing are recommended to take a science SAT Subject Test, preferably in chemistry. Students whose native language is not English are strongly encouraged to take the Test of English as a Foreign Language, or TOEFL.