<p>Double-majoring at the undergraduate level is fairly common at most schools, and in many cases, is more normal than having a single major (such as at CWRU, where even triple-majors outnumber single-majors). The pros would be that it allows you to follow multiple interests and put off resigning yourself to a specific career path right away, and thus enhances theoretical job choice. But the con is that it often means fewer electives and it is harder to ensure you can fit in all the required classes. Moreover, dual majors, while having broader options, don’t have any sort of edge when applying for a specific job.</p>
<p>At the graduate/professional level, it is very rare. This is true for a few reasons. First, the cost is quite a bit higher than the typical undergraduate education, and thus is a roadblock for many. Second, it is quite hard to fit everything in, while still being able to financially support yourself or maintain sanity. Third, for many schools, you apply to the two programs individually and must be accepted into both. This is quite hard, especially with the example you gave, as MBA programs typically demand substantial work experience, whereas law school does not. Finally, most people see little value in the additional degree, or consider it to be less than the cost, in terms of money, time, strain, and so forth.</p>