I always prefer writing images to two cards. Failures happen (not to me yet, fortunately). But just like I back up my computer regularly, I prefer having two copies of a photo from the moment I take it.
Not possible with the R8 which I took to Italy recently, in that case I copied the files off the SD card each night (to the 13" MacBook Pro I brought with me, and from there to a USB flash drive as well, the latter to guard against loss/theft).