Enclosure Math: IP42 + IP64 = IP??

Question: If you took an IP42 rated enclosure and put it in another slightly larger box rated IP64, do you think that would provide more protection than a single IP65 box?

Not on paper, but under real testing.

Guessing encouraged.

"If they can design a black box to survive a plane crash, why not make the whole plane out of the black box?" - Jerry Seinfeld

Ok quick, what's the IP rating of a flight recorder?