Dual storage card slots are more about options and feature creep. I don't think the presence of dual storage cards slots should be taken as a hint that cards might fail. Options including backup to the second slot, overflow to the second slot, JPG in one slot/NEF in the other, and still images in one slot/video in the other, cover the variety of practical storage card uses in a dual slot camera.
Two CF cards failed on me several years ago. They were a pair of older 512MB and 1GB cards with literally thousands of usage cycles. They'd simply exceeded (by far) their rated number of writes. I had the cards for eight years. CF media manufactured somewhat more recently are even more reliable. I cycle a dozen CF cards (4GB, 8GB, 16GB) in & out of my D700 (over 60,000 shots). I've been using most of the same dozen cards since I purchased the camera over three years ago. None of the cards have ever failed or thrown an error.