The Two Card Shuffle
Or " How I Learned To Save My Hide ".
Many modern cameras come with two memory card slots incorporated in the body. This is no mean feat in some cases - if you are going to use the larger CF-style card it has a certain bulk and the volume of the rack into which it slides uses up a fair bit of the inside of the camera. Were you to design a camera with two CF card slots it would need be a larger body...and by and large the ones that do, are. These can be viewed as professional devices and professionals carry heavier weights.
Then there are cameras that have a hybrid style - they have a CF slot for that size and an SD slot for the smaller -sized SD, SDHC, and SDXC cards. In some cases I think that the designers might have started with one of the larger bodies and just used what was a CF bay plugged up for SD. No real overall reduction in body size.
Finally, we have the cameras what have two SD slots - and here the rack structure can be smaller and they can fit in with smaller body sizes. This can be a real selling point if people are trying to reduce bulk and weight in what they carry or if they have smaller hands. I often think a number of cameras have succeeded or failed on their body feel without the prospective buyers really being able to say why they liked them or disliked them.
But two cards - what do you do with two cards? You confuse yourself, if you're not careful, but you save yourself grief if you are clever. Two cards can:
a. Record the exact same thing in exactly the same way. Two RAW images or two JPEG images at the same time. If you do not trust that one card will do it, or know that you wish to put one card away physically for safe storage while the other one is re-used. Or if you know that one would be confiscated by the border police...
b. Record the image in RAW on one and JPEG on the other. This is the case where some cameras have the second slot for a much more faster card. It means that your camera might be able to cram things into the two slots - SD for JPEG and XQD or some other fast card for the RAW - and get an overall faster firing rate than trying to put both file types into one card. If you need speed you do strange things.
c. Record video on one and reserve the other for stills.
d. Record in sequence. All the previous options have been predicated upon parallel recording - if you elect to make it serial, you can carry twice the memory capacity in your camera. This is a matter for philosophical debate if you are trying to cram an entire trip onto only the memory cards in one camera, but becomes a matter of technical necessity if you have a camera that is going to be posted up somewhere to record time lapse work for a long time and need a lot of memory.
It will pay any photographer who gets a new camera with dual slots to sit down and read the factory manual about the numbering and distribution of the files. Indeed, if there is an aftermarket book - like the Rocky Nook series by Rico Pfirstinger who deals with Fujifilm cameras - that can also be a wise purchase. There are lots of options in what you do to the files that can make a difference to how easy they are to work with in the future.
I recently opted to do the wrong thing, after years of doing the right thing, and ended up shooting a fresh job with image numbers that duplicated earlier numbers - I must have poked the wrong setting while camera fiddling. It means that unless I renumber the new job I encounter occasions when the computer can't decide which of the two identical numbers I want and stops until I decide.
Note: the new Pentax K-1 has dual slots for SD-size cards as does the Fujifilm X-Pro2 and the designers have in both cases been wise enough to put the compartment for these around on the right side of the camera. In practical terms this means that if the cameras are locked onto tripods and need a card change, you can do it without unbolting the whole darn thing. This is no small advantage - I wish I had it.