If someone had told me when I first started shooting that I should have a dual-slot camera, I would have been like, “Uhhhh…” Would have had no idea what they meant! Now, I can definitely see the importance of them, especially when shooting weddings.
A dual-slot camera holds two memory cards. I shoot Canon and they have slots for a Compact Flash (CF) card and an SD card. Although my areas of expertise are not other camera manufacturers, I think the only dual-slot bodies that hold two SD cards are Sonys. Anyway, cameras with dual-slots come with the capability to record to both cards simultaneously. Initially, this seems a little redundant…why would you need the same images twice? But when you really think about it, you are ensuring that you have a safeguard against losing any images!!
Could you imagine?! Getting home after a wedding and realizing you are missing 16GB worth of coverage on a client’s WEDDING DAY because a card became corrupt…or was dropped in a puddle…or got chewed by your dog?! That would be catastrophic.
Everyone does this differently…some people record RAW files to both the CF and SD cards, while others record RAW to one and JPEG to the other. You are able to tweak those settings in your camera. My personal stance: why not record RAWs to both? If you do end up losing one card worth of memory…why would you risk having to edit hundreds of JPEGs and, therefore, sacrificing your editing consistency? In my experience, recording RAW images to both cards does not slow down the speed of your camera as long as your cards have a high write speed. This is something you should check before purchasing your memory cards.
Moral of this story: Be safe. Not egregiously in error by losing someone’s wedding day images. Buy the dual-slot camera.