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Some cameras (SBIG) have two chips on the same plane. One for imaging and one for guiding. You can't guide on the same CCD you are imaging with.
When you guide you take a photo of a star, then take another one and so on in short intervals on time. A piece of software on the computer compares the images and how much the star has moved then moves the mount back to re-center the guide star. So you're still taking a lot of short exposures photos.
When you image you expose the CCD for a longer period of time until the wells of the sensor fill up, then download the whole photo to your computer. This process cannot be interrupted once it is started.
This is why you can't do both at the same time and need separate chips.
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