Based off a few online searches, it doesn’t look like there’s an established School of Comm / Medill dual degree program (which is what a double major between two schools is called at NU), but that doesn’t necessarily mean that you can’t do it. I would suggest contacting a university official.
Proof:
http://www.ugadm.northwestern.edu/academics/schools/communication/academic-options-and-areas-of-study.html
http://www.ugadm.northwestern.edu/academics/schools/medill/majors-certificates-dual-degree.html
Dual degrees between Weinberg and an other school are not at all uncommon, fyi.