Northwestern uses a quarter system and encourages secondary majors. I also looked at the website for the requirements for this, the program was actually reasonably doable.
Brown and Rochester have very few non major requirements which make it easy to explore different subjects, I am not aware of how bad scheduling the classes would be, but fitting them on a schedule to study 2 subjects is incredibly simple. Brown also has pass/fail for many courses, although I forgot what the conditions were for this.
Chicago has a lot of overlap among math related majors (like cs or phys) and math. There is also a core that covers a wide variety of courses in differing subjects which is not my favorite thing, but is not awful as far as completing two majors. It also works on a quarter system which is supposedly better for scheduling different classes like Northwestern.