Chance me Northwestern ED [PA resident, 3.85 GPA, 1510 SAT, economics]

???

Unless the other schools with EA have restrictive EA where applying there means not applying ED anywhere (and possibly not applying EA to some set of other schools), then there should be no issue applying ED to one school and EA to other schools. Of course, the applicant needs to be sure that if the ED application results in admission, they will matriculate there, regardless of what the EA applications give.