It is my understanding that Princeton requires you to have 1/3 of your credits from outside your major and 1/3 of your credits to do whatever, and only 1/3 towards your major. Is Princeton unique in this aspect that they emphasize interdisciplinary education? If not, what other schools do this (other than brown, of course). Thank you! 
That’s a very rough description of Princeton’s academic requirements. Like most (if not all) liberal arts schools, a breadth of courses in different distribution areas is required (e.g. math, science, history, language, English, ethics, epistemology), so Princeton is definitely not unique in this way. In fact, one could argue that Princeton emphasizes interdisciplinary study less than its peer institutions, such as Penn. Double majoring is technically not allowed, as the course of study for each major is intensive enough without trying to fit in all of the courses for a second major, as well as courses for certificates and distribution requirements. There is the option for students to make a “independent concentration” that consists of courses from two or three existing majors, but only a handful (less than a dozen) students per class year actually do this.
@DLithium This is not unique to Princeton. Penn is also well known for its heavy focus on interdisciplinary education. Has a very flexible curriculum, makes it very for students to pursue double majors/dual degrees and you can take classes in any of Penn’s undergrad and grad schools. Also as you said Brown has the open curriculum which allows you to explore many different things.