I have done a lot of timing on the D800, and the time to write is approximately additive. Whether that means the cards are written sequentially, or simultaneously but somehow much restrained I do not know, but the effect is as though sequential.
The D4 I did less testing on, but it appears to be the same. I didn't worry about the D4 much as the huge buffer is hard to fill up even in backup mode with a slow CF card.
But it's a serious issue with the D800 if you happen to pick a big-but-slow card for both slots in the D800. I settled on two medium speed cards, and think that's a good compromise. I've got something the same in the D4 (the older XQD series plus a 60 MBS CF card, and have almost never hit the limit in backup mode (which I normally shoot in -- I mean why not, if you have two slots).