Not to.mention that a HS junior has neither been to college nor applied yet.
OP, you don’t need an official double major to take the classes in mktg. You use contacts to get real internships or projects. The employers don’t just say, ooooh, she had a dbl major. They look for how you got real life experiences. You can list lots of relevant details on LinkedIn.
At some colleges, candidate mktg or the promotion of causes is already built into the poli sci curriculum. Or were you thinking of being counsel for a toothpaste mfr? Even then, the law school record trumps. (Or a JD/MBA.)