Im sure some of the older DSLRs had dual slots & you could set them to record to them in different manners - fill one then the next, fill alternately, save images to both cards.
I guess as flash memory has got larger, the perceived need for 2 slots has diminished. I still think it would be a good idea though - 2 cards, instant back-up.
Why do higher end DSLRs have CF rather than SD? Is it faster? It seems to me that CF is more prone to potential mechanical failure as you are inserting pins into holes, rather than SD where the contact is made by sprung fingers on contact patches.
