SAT I and SAT II: Subject Tests are two different exams. They are not administered together. Most colleges require test scores from the SAT I. Additionally, some schools, generally in the upper tier, will ask for SAT II: Subject Test scores. If you want to know whether or not you should take the SAT II tests also, then you should consult the admissions department at your prospective colleges. Furthermore, schools that require you to take additional SAT II tests will likely ask you to take a certain amount and type of those tests, so be sure to ask about this as well. For instance, MIT requires at least one in math (Level 1 or Level 2) and one in science (Physics, Chemistry, or Biology E/M).

you have to register for them separately and take them on different days