I'd say major in one subject, but take a lot of upper levels in BOTH subjects. Even though I was a physics major, I still took many important courses in chemistry... including 2 terms of organic chemistry, and advanced inorganic chemistry (great labs with coordination compounds!). In fact, I would have received a double major if I'd taken two terms of physical chemistry... but the classes didn't fit in my schedule and I thought there would be too much overlap with thermo and quantum (although I bought the text of the P-chem book for reference). You might want to look at what materials science graduate programs suggest for undergraduate applicants to have taken, and tailor your program of study accordingly.