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.
Someone has to ask. Why not major in Materials Science if that's what you want to do?
I've actually applied (in the UK) to a few universities' to study materials science. Almost all said that you needed either physics or chemistry to get in, and go on to study both while you are there. Studying both would be a good idea I guess.
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