No, you don't need to have cards in both memory card slots to shoot with the camera. You just need a single card in one or the other.
If you do use both, you can configure the camera to use them different ways.... 1. save to one card first, then when it's full switch to save to the second card, or... 2. save the same file to both cards, as a backup, or.... 3. save CR2 files to one card and JPEG files to the other. It's up to you.
Until you configure the camera to one of the others, I think it will use #1 by default, any time you install both cards. Even if you configure one of the others, if you fail to install a second card, the camera will just warn you about the empty slot and you can simply click through and go ahead and shoot, if you wish.
By today's standards 30 mb/s is quite slow and 8GB is relatively small. This will probably cause your camera to slow down from it's fastest continuous shooting speed and have to pause to empty the buffer more often. And, if you shoot full size RAW files (Canon's CR2s), I think you'll find that 8GB is only good for around 200 images (24 to 26 image per GB). If you shoot JPEGs only, you should be able to save more images... or if you shoot RAW + JPEG, less. Maybe this is enough for your purposes... maybe not. Personally I sometimes shoot 200 image in an hour or less.
Memory cards aren't expensive these days. 16GB and 32GB cards can be bought for $50 apiece (top quality, well-known brands such as Lexar and Sandisk... others might be even cheaper, if riskier).There are much larger cards available, but personally I'm always a little nervous about putting "too many eggs in one basket". I prefer to use a handful of smaller cards, to one big one. That way if one fails or is lost, I don't lose too many images. And it only takes a few seconds to swap out a fresh card, when I've filled the first one.
150 mb/s (1000X) or faster UDMA 7 (CF) or UHS (SD) cards should give very good performance in your camera. There are even faster available, but the camera won't be able to take advantage of them. For example, faster UHS "II" SD memory will work fine, but only at UHS speeds. One possible reason for faster cards might be if you use a card reader that can take advantage of them, for faster downloads. Otherwise, there's not a lot of reason to buy 300 mb/s, 2000X memory cards, just to use them in this camera.
You just spent about $3500 on the camera... don't cheap out on the memory cards you use in it! I try to stick with well-known, reliable brands such as Lexar, Sandisk and a few others. There are a lot of brands I've never heard of and would be reluctant to risk, even if they are much cheaper.
Also make note of the instruction to format the card in the camera, especially if it's ever been used in another camera. Formatting configures and preps the card for use in a specific camera. There can be compatibility issues if not formatted or formatted & used in another camera.
Formatting will "erase" anything remaining on the memory card, so be sure to copy anything you want to keep off of it before formatting. (Formatting in camera this way doesn't actually erase things, but it marks them to be overwritten.) It may be adequate to format the card in-camera once, prior to the first use in the new camera. But, personally I format every card every time I use them (but I also use them in several different cameras).
Have fun with your new camera!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
