FYI, more common at Northwestern than a Civil Engineering / Economics dual degree is one in Industrial Engineering (IEMS) and Economics. I know its not exactly what you want seeing as Industrial and Civil Engineering are very different beasts, but there is more of a precedent for it at NU. It may or may not be worth looking into. At worst it can serve as a roadmap for planning a Civil Engineering / Econ dual degree for you.
Industrial Engineering: http://www.mccormick.northwestern.edu/industrial/undergraduate-program/index.html
IEMS and Econ program: http://www.mccormick.northwestern.edu/industrial/documents/undergraduate/ie-econ.pdf