I agree with Gumbymom. The question you have to ask yourself is if you would rather go to UCLA and major in applied math, or go somewhere else where you can major in computer science.
At schools where your major affects your admissions chances, it’s usually difficult to transfer into the major.
And universities expect that at 17 you don’t know 100% what you want to major in, so it’s fine to put two majors, one as a backup. Math and computer science are related, but also, you don’t have to look like you’re “passionate” about your major.
It applies to colleges where you have to apply to specific majors. There are some private colleges that make you apply to specific schools or majors - like at NYU, where you have to apply to specific schools within NYU. And there are some public universities where you don’t have to apply to majors; once you are admitted you can choose any major you want.