If you have more then one career , are you more likely to spend more time on one then the other?

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.