Here is a summary of some research on the economic returns to double majoring. But at the same time, I don’t think the benefits of double majoring are high if your goal is to go to law school where the GPA and LSAT matters more than what your major or majors were.
http://www.sciencedirect.com/science/article/pii/S0272775707000659
From the paper, which you can access through your university library, this was stated about the benefit of double majoring for those who later go to grad school: “Even though the rate of double majoring is higher among those who achieve graduate degrees, having a double undergraduate major does not increase earnings controlling for level and/or field of the post-bachelor’s degree.”