In this world, I have three great loves: Astrophysics, Politics, and the Yankees. Because I unfortunately can’t pursue a degree in the Yankees, I’m trying to decide which of my other interests to pursue, if I can’t do both. Is this combination doable, workload wise? And if so, which major should I aim to get more college credits for while I’m still in high school? I have a lot of time, as I’m a rising sophomore, but given how different Politics and Astrophysics are, I want to know how to get myself into the best position possible when I do go to college.
And just a note, I know that there’s no plausible explanation for why I would possibly want to double major in these categories, as there’s not really a career that allows overlap, but I honestly don’t know how I’ll ever decide between them. I’m just trying to buy myself more time to decide and feel each one’s career path out.
Of course it’s possible- but it’s easier at colleges/universities that have more flexible GenEd requirements (such as Brown, Vassar, Bard, Beloit, URochester, etc). If you have a state flagship or other large universities you already know that you are interested in, check their fine print and see how many of the GenEds you can take during high school - with those out of the way it will be easier to focus on your major subjects. Take your math as far as you can while in high school- that helps a lot with physics.
But: don’t get over focused on your major just now. You don’t have to ‘buy’ yourself more time: you have the time: almost 5 YEARS before you will have to declare. You will have changed as much between now and then as you have between 4th grade and now (which I have to assume is quite a lot!). So, go ahead and the math classes and putz around with gen ed requirements- it can’t hurt- but don’t get too inflexible.
You shouldn’t aim to get credits in either major right now. A lot of colleges will only accept a very limited number of credits in your major from anywhere else. Instead, if you can knock out lower-level general education requirements like collegemom3717 mentioned - English, math, a social science here or there - that would be the best way to go. For example, get the calculus sequence out of the way if you are advanced enough in math, because those are cognate courses for physics.
Also, I wouldn’t assume that there’s no careers that allow overlap! Think about science policy careers, for example. There are Congress members who have scientists that advise them on science issues because they don’t know enough themselves to make policy decisions about the space program. There are advisers to the White House and other administrative organs of the government that do the same thing. There are program managers and administrators within NSF and other federal agencies that determine how much research funding goes to certain scientific pursuits, and a lot of that is tangled up with politics. There are positions at NASA that involve liaising with the government about funding and about what research is allowed.
With commercial spacefaring companies like SpaceX becoming bigger, those companies may also need lobbyists and advocates with their governments who know both the science and politics, and who can make sure they’re allowed to operate and do research.
And that’s only domestically - think about international space policy, for example! Like different countries coming together to build and maintain the ISS, or determining who can do what in what sectors of space.
The lines between fields is not as rigid as most people imagine 
Besides all that…it’s also totally fine to just major in things for the love of them.