You could use a distance learning program, like EPGY. But this seems rather pricey just to avoid feeling idle. Surely you can come up with something interesting to do than doing work that you can easily do in the fall. How about reading some books on math history or some aspect of math outside the usual course progression? Or take a course from www.artofproblemsolving.com?
Or you can skip math altogether and read some books you've been meaning to get to, study a foreign language, find a sport, volunteer, get a job, learn an area you are unlikely to take a course in at college ...
If you do self-study (and I agree learning both subjects would be difficult in one summer), make sure that your college will allow you to test out of the subjects or self-certify prerequisites. It might be rather boring to take the same course again. It might be to your advantage to use whatever textbooks your college uses.
From just my son's experience with EPGY (for which his college would NOT give him credit -- he was just able to skip taking the courses again), I think it would be easier to self study multivariate. He is good at math (he took BC Calculus in 9th grade and probably could have accelerated more if schools had been more cooperative), but felt having a teacher in linear algebra was very useful.