In my experience, another major is usually better than having a minor in an area of study, but it doesn't mean that minors aren't helpful. Your best bet would be to contact potential employers and ask them whether or not a minor helps when reviewing candidates for a position. Since you're thinking of teaching at the high school level, you could even start with your own high school (hopefully they're more than willing to help out!) Getting a double major is tough though, and will require very careful scheduling of your classes so as not to extend your stay past 4 years. Make sure you get your academic or course advisor involved in the conversation as well.