Permethrin is a suspected human carcinogen.
DEET is an acetylcholinesterase inhibitor. It's considered safe enough to use on human skin (in low concentrations).
A mixture of a permethrin-based insecticide and a DEET based one shouldn't be any worse than the permethrin one alone.
We'd have to know more about what kind of risk you're talking about to give you a better answer than that.
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