Double the Storage

From everything I can gather, the Kodak DCS-14n, a Nikon N80 converted to a full-frame digital SLR, was the first one to offer dual card slots. It sported a Compact Flash slot, which had been the typical storage medium for digicams back in those primitive days, as well as a slot for the then-new Secure Digital format.

In 2004, Canon added the same feature to the second iteration of their EOS-1D pro body. Like the Kodak, the Canon allowed you to write to the cards either sequentially, making the second one a "spare magazine" of sorts, or you could write to them both at the same time, which offered redundancy in case a card was damaged or lost. Unlike the Kodak, you couldn't take advantage of the (at the time) much faster writing speed of the CF card by telling the 1D Mark II to save .RAW to the CF and .JPEG to the slower SD.

I have no idea why the initial two-card configuration happened. I am guessing that it was because SD cards were seen as the wave of the future, as indeed they have been in non-professional cameras. Pro photographers have a bunch of accumulated infrastructure in card readers, spare cards, et cetera, and remember that back in the early Aughties reasonably-sized memory cards were a not-inexpensive investment.

Indeed, to this day my Canon 5DS retains the "1 CF slot & 1 SD slot" configuration.

Somehow, though, "dual card slots" became one of the ways to signal that a camera was a "pro" body, or at least had pro aspirations. Wedding photographers especially seem to love them some dual card slots.

So when Nikon dropped the D3 in 2007, it also had two card slots...but they were both Compact Flash. (And the D7000, which was decidedly straddling the line between pro and consumer features, had a brace of SD card slots.) Format upgradeability is obviously not the aim anymore; it's just a thing that's done because that's how we do things.
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