Today many professional and prosumer cameras come with two memory card slots. By having the ability to put two memory cards in your camera at once you can extend your camera’s capabilities. Having two memory card slots at your disposal means you can do two things: 1. you can set your camera to write to one card at a time; when the first card fills up it will continue to write to the second card. This affectively doubles your storage space and reducing the number of times you need to change your card. 2. You can set your camera to write duplicate copies of the image each time you press the shutter button; this is helpful because memory cards fail but it is unlikely that two cards will fail at the same time. When writing to both cards you can set your camera to write two raw images, two jpg images or a raw image to one card and a jpg image to the other. It should be noted that when you write to two cards at the same time, your camera will write more slowly than if it were writing to just one card at a time. While most high end cameras allow you to fire shot after shot after shot, you may find the camera will stall after 15 or 20 shots when you are writing to two cards.
It should be noted that when writing to two cards at once, your camera will write as fast as the slowest card you have in your camera. Therefore it is recommended that both cards are equally fast.
As you can see there are many advantages to having two card slots. The disadvantage is price. Cameras with two slots tend to be much more expensive and bit larger. Whether you need it depends the kind of photography you do most, for instance if you are shooting things where it’s difficult to get reshoot such as sports or if you like to travel to remote locations; it’s helpful to have a back up. What this comes down to is how much you value the time and effort you put into your photography. The decision is yours.