Your advice is good
if a student really knows what they want to do when they start. Unfortunately, most students change majors, once or twice, but sometimes more. I suspect PoliSci/IR was relatively easy to double major, as there should be lots of overlap in major courses, i.e. courses that not only fill elective requirements for one or both, but dual listing of courses for the major. In our daughters case coming up, looking at BM Music Performance/BA History or English, there will be less overlap, but electives for one can sometimes be the required major courses for the other.
I didn't double major, but I have to say I was always careful to make sure I had my requirements fulfilled or waived early on. The most important thing a student can do is to get the current college catalog and read it carefully. Everyone concentrates on the viewbooks and fluff, but the meat is in the catalog.
I agree about looking at a school's catalog, not only the courses but the required (core) courses for graduation. One caution about college catalogs: about 20% of the courses listed are rarely taught or have not been taught in years. I kept looking for one particular course at registration only to find out it had not been taught in five years.