Separate Lives (1995)

Lauren Porter (Linda Hamilton) is a respected psychology professor with a troubled past who believes that she might have developed multiple personalities. In order to find help in determining her condition, Lauren recruits a student, ex-police officer Tom Beckwith (James Belushi), to follow her and observe her activities. It soon becomes clear that Lauren does indeed have another personality -- one who happens to be a hedonistic temptress involved in plenty of dubious dealings.