In regards to using sunscreens with different levels of SPFs, Dr. King says: "It's ok to use different SPFs, but keep in mind that improvement over SPF 30 is marginal, and be sure to avoid any skipped areas. If they overlap, the resulting SPF is the higher one at best — it's not a sum of the two, and it's possible that one may dilute the other, resulting in an SPF lower than the higher one."
She says that it's also important to realize that both UVA and UVB rays cause skin cancer, but the SPF actually only tells you about the product's ability to block UVB rays, not UVA rays.