I actually have doubts that those two majors are "related". It sounds like a statement from someone who knows only a little about either. The thought patterns and insights are very different. It isn't a question about accumulating things, but about how the things fit together.
Note that academia is about specialization - more and more the farther you go. But at the beginning it is natural (and good) to be undecided and hence flexible.
You can do well (most places) with a double major but you have to work a lot (a lot) harder. Mathematics itself is a flexible position from which to go forward, as it is applied in many fields. The most flexible field (IMO) is actually philosophy, but that is a different discussion.
I'll note that you don't have to decide early to accept one field and reject another (or others). If you develop a career you can branch out into other things that interest you. But you need to start from a solid base.