Do you mean concurrent careers or sequential careers? If you're doing two things at once, yes you will often have one "major" and one "minor". As it's hard to do two things really well at once, most people never really accomplish this kind of multitasking. However, it's often the case that people will have multiple careers over their lifetime, one after the other. They're usually related by some common thread.
I think it depends on how you define career. You could definitely have a career that "pays the bills" and then work with non-profit organizations in your free time. However, if you mean 40-50 hours for both in a week then that would be very tough.