For high school level teaching, many do exactly that, dual major. Others go the alternative route whereby they majored in one field and then went and got more education specific to teaching on top of that while completing a state certified alternative education program.
As a veteran, professional teacher, I have to admit to advantages and disadvantages to both from an employee perspective. Those who major in education and/or a duel spend their final semester student teaching and go straight into a job. For those of us who "broke" into education via the alternative route, our first year is "probationary." Not only is the certification program management and assessment team micro-dissecting everything you do, so is the administration. If stronger metal is forged in the fire, the later is definitely the way to go.
From a student perspective, no one can effectively teach anything they don't know very well themselves, especially to students falling on the gifted or functional levels.. Sadly, the best component making up any teacher can't be taught. Teachers have to honestly love children, want what's best for them especially when it's not popular, have high expectations in the face of some pretty stiff odds, and never ever give up on any student.