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.