I've written this many times before, but again, then.
From what you can dechiper from Canon's information, it's like this.
Cameras have a Digic something CPU, or two of them (depending upon frame rate) to do the following:
Image processing from sensor.
Memory card and buffer memory control.
Rear LCD control.
Contrast detection and face recognition AF in live view.
Lighting type evaluation (5D Mark II & 50D).
Then there's a sequence control CPU, rsponsible for everything else. This is a RISC-based processor at about 40 MHz, of a different kind than the Digic chips, optimized for image processing as they are. This CPU takes care of the following:
Handling buttons and dials.
Shutter and mirror control.
Exposure metering and calculations.
AF metering and calculations.
Then some high end cameras, like the 7D, 1D Mark III, 1DX, 5D Mark III and some more, have a dedicated AF processor. This is also a RISC-based CPU, not Digic type, running at around 40 MHz.
The 1DX is the first one having a dedicated processor also for exposure metering. In this case it's a Digic type CPU, but that's becuase it also has to do image analysis, finding and tracking faces in the image etc.
Thus cameras with the "normal" dual Digic configuration (7D, 1DX etc.) don't use these for normal AF and other such stuff, but they are dedicated to image processing.