A second major in Mathematics could definitely be an asset for applying to graduate school in Philosophy, but whether it is, or how much it could be, depends a great deal on what specialization you suggest you are likely to pursue in your application statements. If you anticipate specializing in (a) an area where mathematical methods are used (e.g. logic, formal epistemology, or decision theory), or (b) a "philosophy of x" area where the subject analyzed involves math (e.g. philosophy of math, philosophy of physics and a number of other scientific fields, philosophy of computing), it could have significant value. If you plan to pursue areas where neither of these apply (e.g. virtue ethics, wittgensteinian philosophy of language), the second major would serve only to support your academic seriousness. Meanwhile, there are other ways of demonstrating academic seriousness. In any case, it is certainly not essential.
The question then is a matter of relative benefit, in comparison with what else you could be doing with that time. Your first priority, in terms of applying to graduate school, should be doing very well in Philosophy courses, helping (to the extent you can) philosophers who may write letters for you get to know you intellectually, and producing an excellent, well-revised writing-sample essay. Think about whether the Math major interferes significantly with any of that. If it interferes mainly with the last, consider whether you can continue to work seriously on your writing-sample and get feedback on it, after graduation. To the extent that your college or university does not have a high academic reputation or your recommenders do not have reputations as philosophers, it becomes increasingly important to spend time in addition to coursework revising your sample essay.
However, you might also consider, in light of the possibility that you come to be in the majority of students starting a PhD in Philosophy who don't end up in an academic position in Philosophy, whether the Math major might have significant value (along with your work in Philosophy) in your application to another career. Given your interest in Math, I suspect that it might. Indeed, I know cases like that—of Philosophy PhDs pursuing non-academic careers—where an undergraduate major in Math has been a career-asset, at least at the beginning of their careers.