so nobody knows the answer to my question? and no i can't just go to the store, i'm on a remote lake and the marina only has spf 50 and higher.
check the ingredients. if they are both based on the same chemicals, then yes mixing them will get you a sunscreen that is about halfway between the two. if they're based on diff. chemicals it prolly will anyways.
honestly, though. at this latitude, with spf 30 you're not hardly going to tan at all, even if out all day. at least i wouldn't.