The SAT II exams, unlike the SAT I exams, offer the option of Score Choice. Score Choice allows you to take an SAT II more than once but have a college only see the higher score. Scores for the SAT I, on the other hand, may not be suppressed from colleges. Many colleges offer to only consider your highest math score and highest verbal score, but ETS will send them every math and verbal score.
The SAT II exams are offered on the same dates as the SAT I, except that there is no SAT II offered in March. You cannot take both exams on the same day. You can take up to three SAT IIs on the same day, although we do not recommend taking more than two on a given day. In general, you should plan on taking an SAT II around the same date as your final in the equivalent high school course in order to eliminate studying the same subject twice.