WM8775SEDS: Rear Microphone, Line input, Aux input (Creative I/O bay).

WM8782S: Front panel Microphone.

The reason to have two ADC is to be able to use two input port independently. For example, you can input Front panel Microphone and Rear line input and do the mixing or recording. Moreover, in Vista, there is a separate driver for each input jack (microphone driver, line in driver, etc…) and therefore, it is recommended to use separate ADC.