As FS mentions, this information is in your Normal Procedures - Autoflight section. It is also mentioned in the Systems Description of your FCOM stating (paraphrased) that for dual channel A/P operation, 2 AC generators are required. For Autoland, we know this is due to redundancy requirements.
In your configuration, if you have only one AC power source, but have both FCC's powered, your electrical configuration has the Main AC BTB closed. In this config, there is no redundancy in cases of a further AC/bus failure.
Grab an engineer and ask to have a look at the AC power schematic for your aircraft. The operative phrase "dual redundancy" should become clear to you.
I agree, the books fail to provide a detailed, direct connection from Autoland requirements to ELEC reconfiguration scenarios, however an overall systems understanding will make that light appear above your head, 172.
