Can using two sunscreen products mess one another up and diminish your level of sun protection?

No, but...: ...Why would you want to apply two different sunscreens? Pick one (preferably with a high spf rating), and stick with that. However, sunscreens and insect repellants do interfere with each other; so if you want to use both, apply the repellant first, wait an hour, then apply the sunscreen. Reapply sunscreen every 90 minutes or so, but do not reapply the insect repellant.