Honestly, I have to agree with the minor suggestion, versus a dual major, unless your education is being funded and you're not worried about loans.
As far as to pick, practicality would suggest mathematics, but I'll provide a different approach. The best developers are often people who don't narrow their focus to one practice, but rather, have a specific skillset, and then branch out in hobby or passion that also keeps them engaged in a multi-disciplinary way. The most interesting and successful developers and IT professionals I've worked with often have creative outlets that they pursue, and these often help inform how they approach their IT/Tech-related work.
You might think development and coding go hand-in-hand with math, but you also might find that a creative replacement for math is music - with its similarities in mathematical complexities, but the expansiveness to creativity. Or abstract art with it's counter-point in geometry and understanding how to visualize objects in three-dimensional space is useful when running through problems. There's a lot of ways to go approach your career, and it shouldn't be about fitting inside a box.
As an accomplished IT Project Manager and Operations Stategist - my degree is a BA in English with a focus on creative writing. It has helped provide a unique lens through which I (opinion) better communicate, and don't silo myself into one avenue. It makes me a more relational thinker, versus linear thinker - even though I'm primed to also think linearly through processes.
So, have a core focus, but unless it's your only interest, I'd recommend that if you want a second major, make sure it's something you're equally passionate about. Life's too short to only focus on what makes us "successful" without also figuring out what brings us joy.