I'd happily take a 'pro' camera with one slot, I paid $1,000's for the early 1 series that all had one card slot, you can pay $10,000's for current medium format digital cameras with one card slot.
I had way more issues back in the days of film with lost and damaged rolls than I have ever had with digital files.
(Full disclosure--everything I own has one slot) I suspect if my living depended on it, and I *ever* had a card go toes-up, or ever saw someone else go through it--I'd want that second slot.
But I think I'd want them to both be the same card type, to keep life simple. I'd hate to have to start buying Compact Flash (seriously? Bulky, with pins that bend if you look at them crosseyed?) just to use the feature.